(Bloomberg) — Four commercial cargo vessels were subjected to sabotage attempts early on Sunday near the coast of the United Arab Emirates, an area close to the world’s most important oil transport route, adding to regional friction as the U.S. increases pressure on Iran.

The UAE’s foreign ministry said the incident, which took place near its territorial waters in the Gulf of Oman, was a “serious development” and was being investigated with local and international parties.

No one was hurt and no fuel or chemicals were spilled, the state-run WAM news agency quoted the foreign ministry as saying. Work at the nearby UAE port in Fujairah was not interrupted.

The UAE did not identify the alleged attackers nor give details of what happened but asked the international community to stop any parties “attempting to undermine maritime traffic safety and security,” WAM said.

The incident comes at a time of heightened tensions in the Gulf. The U.S. has deployed an aircraft carrier, bomber planes and defense missiles to the region amid rising tensions with Iran, which has threatened to block oil shipments through the Strait of Hormuz if the U.S. succeeds in halting its energy exports.

Antagonism between the countries worsened this month since Trump eliminated exceptions to U.S. sanctions on Iranian oil. The Islamic Republic responded by threatening to scale back its obligations under the 2015 nuclear deal.

Strait of Hormuz, the World’s Most Important Chokepoint for Oil

The Strait of Hormuz connects the Gulf to the Indian Ocean. Iran is to the north and the UAE and Oman to the south. It is the single most important waterway for global trade in crude oil, with tankers hauling about 17.5 million barrels passing through it daily, Bloomberg tanker tracking shows.

That’s equal to about 40% of all the crude traded internationally. All oil exports from Kuwait, Iran, Qatar and Bahrain, more than 90% of those from Saudi Arabia and Iraq, and 75% of shipments from the UAE pass through it.