President Joe Biden on Monday kicked off the first full day of his trip abroad with a London visit aimed at bolstering the US-UK "special relationship" -- including his first meeting with King Charles III since the monarch's coronation -- on the eve of a high-stakes summit with NATO leaders.

Biden arrived at 10 Downing Street and was greeted by Prime Minister Rishi Sunak ahead of discussions on a range of issues, including Ukraine, a topic on which the two leaders have closely coordinated. Their meeting comes after the US announced Friday that it will be sending cluster munitions to Ukraine for the first time, a rare topic on which the US and United Kingdom publicly disagree.

The UK, Sunak told reporters Saturday, is "signatory to a convention which prohibits the production or use of cluster munitions and discourages their use."

Sunak continued, "We will continue to do our part to support Ukraine against Russia's illegal and unprovoked invasion, but we've done that by providing heavy battle tanks and most recently long-range weapons, and hopefully all countries can continue to support Ukraine."

National security adviser Jake Sullivan downplayed any concern that Biden's decision to send cluster munitions would present any "fracture" with allied countries that oppose the use of such equipment, suggesting that Sunak was stating a "legal position" as he highlighted broader US-UK unity.

"The prime minister stated the UK's legal position, that they are a signatory to the Oslo Convention. The United States is not. That being a signatory means discouraging the use of these weapons. He fulfilled his legal obligation, but I think you will find Prime Minister Sunak and President Biden on the same page strategically on Ukraine, in lockstep on the bigger picture of what we're trying to accomplish and as united as ever, both in this conflict and writ large," Sullivan told reporters aboard Air Force One Sunday.

Sullivan noted that the US has not received any negative feedback from NATO allies regarding the decision.

"That will be repeated, in my view, with all the leaders of the alliance. I do not think you will see fracture, division, or disunity... as a result of this decision. Even though many allies - the signatories to Oslo - are in a position where they themselves cannot say, 'We are for cluster munitions.' But we have heard nothing from people saying this cast doubt on our commitment, this cast doubt on coalition unity, or this cast doubt on our belief that the United States is playing a vital and positive role as leader of this coalition in Ukraine," he said.

A Defense Department release on the US' latest equipment drawdown also said that the decision was made following "extensive consultations with Congress and our Allies and partners."

While it will be Biden's first trip to No. 10 Downing Street as president, this will mark the sixth meeting between Biden and Sunak in six months, Sullivan said, "So I think you can consider this more the continuation of a long-running conversation than kind of a formal meeting."

"They will have the chance to compare notes going into the Vilnius summit, to talk about the issues on the agenda in Vilnius, to talk about progress in the war in Ukraine," Sullivan told reporters aboard Air Force One, in addition to other issues like China, climate, technology and artificial intelligence.

The president will then mark his first in-person engagement with King Charles III since he ascended to the throne, visiting Windsor Castle for all the pomp and circumstance of an arrival ceremony and honor guard inspection, a one-on-one meeting with the monarch and a discussion on climate.

It will be a closely watched moment for how King Charles balances his traditionally apolitical role with a cause he is passionate about that has become a signature priority. Biden has called climate change "the existential threat to human existence as we know it."

Biden, Sullivan told reporters, "has huge respect for the king's commitment on the climate issue in particular. He has been a clarion voice on this issue and more than that, has been an actor -- someone who's mobilized action and effort. And so the president comes at this with enormous goodwill at this relationship," Sullivan said, calling Monday's engagement an opportunity to "deepen the personal bond" and "harness their shared interest in trying to drive climate progress and climate action."

Biden, King Charles and special envoy for climate John Kerry, Sullivan said, will meet with "private sector leaders" at an event "focused on how we bring private capital off the sidelines in service of our climate objectives, the deployment of clean energy, the mitigation of carbon emissions, and help to developing countries in adapting to the impacts of climate change."

The group will discuss barriers to private investment as Biden will encourage those in attendance to "step up to their responsibilities," while also highlighting public investment, Sullivan said.

In keeping with US tradition, Biden did not travel to London for the coronation, but first lady Dr. Jill Biden and granddaughter Finnegan Biden attended the ceremony. Both the president and first lady did make the trip across the Atlantic for the funeral of Queen Elizabeth II last year.

Later Monday, the president departs London for Vilnius, Lithuania, where NATO leaders will gather for critical meetings amid the war in Ukraine and last month's failed coup attempt in Russia, posing the biggest threat to global stability for the alliance in recent history.

Following the NATO Summit, Biden travels to Helsinki, Finland, where he will offer a notable show of support to Nordic countries during a summit with the leaders of Finland, Sweden, Norway, Iceland and Denmark.