Maritime disputes across the vast South China Sea have ratcheted up in recent years as an increasingly assertive China militarizes disputed islands and confronts its regional rivals over their competing claims in the strategically important and resource-rich waterway.

Bracketed by China and several Southeast Asian nations, parts of the vital economic passage are claimed by multiple governments, with Beijing asserting ownership over almost all of the waterway in defiance of an international court ruling.

Over the past two decades, China has occupied a number of obscure reefs and atolls far from its shoreline across the South China Sea, building up military installations, including runways and ports.

Competing claimants, such as the Philippines, say such actions infringe on their sovereignty and violate maritime law.

And the United States agrees, regularly sending its Navy destroyers on freedom of navigation operations close to contested islands, leading to fears that the South China Sea could become a flashpoint between the two superpowers.

Here's what you should know.

Why does the South China Sea matter?

The 1.3-million-square-mile waterway is vital to international trade, with an estimated third of global shipping worth trillions of dollars passing through each year.

It's also home to vast fertile fishing grounds upon which many lives and livelihoods depend.

Much of its economic value remains untapped, however. According to the US Energy Information Agency, the waterway holds at least 190 trillion cubic feet of natural gas and 11 billion barrels of oil.

Who controls those resources and how they are exploited could have a huge impact on the environment. The South China Sea is home to hundreds of largely uninhabited islands and coral atolls and diverse wildlife at risk from climate change and marine pollution.

Who claims what?

Beijing claims "indisputable sovereignty" over almost all of the South China Sea, and most of the islands and sandbars within it, including many features that are hundreds of miles from mainland China. The Philippines, Malaysia, Vietnam, Brunei and Taiwan also hold competing claims.

In 2016, an international tribunal in The Hague ruled in favor of the Philippines in a landmark maritime dispute, which concluded that China has no legal basis to claim historic rights to the bulk of the South China Sea.

China has ignored the ruling: Manila says Beijing continues to send its maritime militia to Mischief Reef and Scarborough Shoal in the Philippines' exclusive economic zone.

In the southern portion of the sea is the Spratly Island chain, which Beijing calls the Nansha islands. The archipelago consists of 100 islets and reefs of which 45 are occupied by China, Taiwan, Malaysia, Vietnam or the Philippines.

In the northwestern part of the sea, the Paracels -- known as the Xisha islands in China -- have been controlled by Beijing since 1974 despite claims from Vietnam and Taiwan.

China's ruling Communist Party also claims self-governing Taiwan as its own territory, despite having never controlled it.

What does China's naval build up mean for the sea?

China has built the world's largest naval fleet, more than 340 warships, and until recently it has been regarded as a green-water navy, operating mostly near the country's shores.

But Beijing's shipbuilding reveals blue-water ambitions. In recent years it has launched large guided-missile destroyers, amphibious assault ships and aircraft carriers with the ability to operate in the open ocean and project power thousands of miles from Beijing.

In addition, Western marine security experts -- along with the Philippines and the United States -- claim China controls a maritime militia that is hundreds of vessels strong and acts as an unofficial -- and officially deniable -- force that Beijing uses to push its territorial claims both in the South China Sea and beyond.

The US is not a claimant to the South China Sea, but says the waters are crucial to its national interest of guaranteeing freedom of the seas worldwide.

The US Navy regularly conducts freedom of navigation operations (FONOPs) in the South China Sea, saying the US is "defending every nation's right to fly, sail, and operate wherever international law allows."

Beijing denounces such operations as illegal.

What has China built in the sea?

Most of Beijing's military buildup is concentrated along the Spratly and Paracel island chains, where sustained land reclamation saw reefs being destroyed first and then built on.

Chinese vessels have been known to encircle various atolls and islets, sending dredgers to build artificial islands large enough to harbor tankers and warships.

"Over the past decade, the PRC has added more than 3,200 acres of land to its seven occupied outposts in the Spratly Islands, which now feature airfields, berthing areas, and resupply facilities to support persistent PRC military and paramilitary presence in the region," US Deputy Assistant Secretary of Defense Lindsey Ford told a House subcommittee earlier this week, referring to China by its official acronym, the People's Republic of China.

Beijing's military construction sped up in 2014 as it quietly began massive dredging operations on seven reefs in the Spratlys.

Since then, Beijing has constructed military bases on Subi Reef, Johnson Reef, Mischief Reef and Fiery Cross Reef, fortifying its claims on the chain, according to the Asia Maritime Transparency Initiative at the Washington-based Center for Strategic and International Studies.

Those facilities, according to Ford, are now bristling with some of China's most advanced weaponry, including stealth fighters.

"Since early 2018, we have seen the PRC steadily equip its Spratly Island outposts — including Mischief Reef, Subi Reef, and Fiery Cross — with an increasing array of military capabilities, including advanced anti-ship cruise missiles, long-range surface-to-air missile systems, J-20 stealth fighter jets, laser and jamming equipment, and military radar and signals intelligence capabilities," she said in a prepared statement.

China installed exploratory oil rigs in the Paracels in 2014 that sparked anti-China riots in Vietnam, a competing claimant.

More recently, cruise ships have taken Chinese tourists to the militarized reefs.

Why are tensions rising again?

Under President Ferdinand "Bongbong" Marcos Jr, the Philippines has taken increasingly assertive steps to protect its claim to shoals in the South China Sea, leading to several confrontations with Chinese vessels in waters off the Philippine islands.

They include standoffs between Chinese coast guard and what Manila says are shadowy Chinese maritime militia boats and tiny wooden Philippine fishing vessels; Chinese water cannons blocking the resupply of a shipwrecked Philippine military outpost; and a lone Filipino diver using a knife to sever a massive floating Chinese barrier.

"These recent incidents in the past year shows that China has become increasingly aggressive and confident in its actions against smaller countries like the Philippines. They're beginning to cross certain lines," said Jay Batongbacal, a maritime expert at the University of the Philippines.

The Philippine Coast Guard says it remains "committed to upholding international law, safeguarding the welfare of Filipino fisherfolk, and protecting the rights of the Philippines in its territorial waters."

China's Foreign Ministry has defended the behavior of its vessels in the waterway and said Beijing will "firmly safeguard" what it views as its territorial sovereignty.

What are the global implications?

Since taking office in 2022, Philippine President Marcos Jr. has taken a stronger stance over the South China Sea than his predecessor, Rodrigo Duterte, amid the wider power struggle that has been playing out in the region for years.

The South China Sea is widely seen as a potential flashpoint for global conflict, and the recent confrontations between Manila and Beijing have raised concerns among Western observers of potentially developing into an international incident if China, a global power, decides to act more forcefully against the Philippines, a US treaty ally.

Washington and Manila are bound by a mutual defense treaty signed in 1951 that remains in force, stipulating that both sides would help defend each other if either were attacked by a third party.

Marcos has strengthened US relations that had frayed under his predecessor, with the two allies touting potential future joint patrols in the South China Sea.

As the partners held their largest military exercise in April 2023, China's Ministry of Foreign Affairs warned that US-Philippine military cooperation "must not interfere in South China Sea disputes."

The US, however, has condemned China's recent actions in the contested sea and threatened to intervene under its mutual defense treaty obligations if Philippine vessels came under armed attack there.

"The increasingly frequent run-ins between China and the Philippines speak to the new Marcos government's willingness to stand up to Chinese bullying and coercion," said Gregory Poling, director of the Asia Maritime Transparency Initiative.

"Part of that is certainly attributable to the closer US-Philippines alliance which helps given Manila the confidence that Beijing will be deterred from overt military force lest it invoke the US-Philippines Mutual Defense Treaty."