Singapore’s detentions of oil and chemicals tankers have surged since early last year, highlighting growing concerns over the environmental and safety impact of an expanding fleet of aging vessels plying major shipping lanes.

The city-state has held 33 ships for failing safety inspections so far this year, the same as for the whole of the decade through 2019, according to figures from Tokyo MOU, a regional port control organization. April saw nine detentions, the most for any month since at least 2010.

The dramatic increase in failed inspections in Singapore fits with a regional trend of rising detentions across Asia since Russia invaded Ukraine early last year. The invasion and subsequent sanctions have prompted a rapid expansion in the dark fleet of tankers sailing under the radar, transporting sanctioned oil to buyers in China, India and elsewhere.

Those ships often operate with sub-standard insurance, and are usually past an age at which they would normally be sent for scrapping. A recent explosion on a tanker capable of carrying around 700,000 barrels of oil off the coast of Malaysia brought home the dangers these vessels can pose. Luckily, the Pablo, built in 1997, was nearly empty.

Read More: An Oil Tanker Ablaze in the South China Sea Is a Global Problem

Singapore, a frequent stopping point en route to north Asia, is one of the busiest oil ports in the world. Most of the crude coming from the Middle East and western Russia will transit through the Strait of Malacca.

The city-state’s Maritime & Port Authority was unable to comment by the time of publication.

Singapore detained 28 vessels last year, with all but two of those happening from June, according to the data from Tokyo MOU. There was a lull in detentions during the pandemic but in the decade through 2019 the annual record was just seven.

China has increased safety checks on older ships calling at its ports in recent weeks, particularly for those that are 15 years or older.