A growing number of ships that received contaminated marine fuel at the major hub of Singapore have suffered power blackouts, according to fuel testing firm Veritas Petroleum Services.

The company identified 34 ships that got contaminated supplies over February and March, of which almost half suffered a failure of the fuel system that led to a “loss of power and propulsion creating a blackout,” Veritas Petroleum said in a notice dated March 31 seen by Bloomberg. The company said the tainted fuel came from two suppliers in Singapore, without naming them.

Veritas Petroleum and Maritime and Port Authority of Singapore didn’t immediately respond to requests for more information when contacted on the matter.

Singapore is the largest marine fuel supplier in the world and sits at the crossroads of a centuries-old trade route that links Asia to Europe, the Middle East and the U.S. Gulf Coast. Marine insurer Gard said in a statement that some of the vessels it insures had suffered operational issues including blackouts, and in some cases ships needed to be towed back to port.

Veritas Petroleum said the ships in Singapore received high-sulfur fuel oil, and that the supplies actually met the required ISO 8217 specifications upon each delivery. However, a more detailed screening of the fuel samples showed they were contaminated with up to 2,000 parts per million of chlorinated hydrocarbons. Contaminations have the potential to damage the engine and lead to costly repairs.

Ships losing power at sea can be an extremely dangerous, increasing the risk of collision or running aground. In 2018, numerous vessels suffered serious technical problems and mechanical damages after receiving contaminated fuel initially supplied in Houston.