ADEN, Yemen, May 1 (Xinhua) -- Multiple commercial vessels anchored at the Houthi-controlled Ras Isa fuel port in Yemen are being forcibly detained despite having proper UN clearance, Britain's Maritime Trade Operations (UKMTO) said Thursday.

"UKMTO has received multiple reports regarding vessels anchored at RAS ISA, Yemen. Vessels have been denied permission to leave despite having UNVIM clearance," UKMTO said in a maritime security alert posted on social media platform X.

The Houthis have demanded that vessels, including commercial ones, "proceed to berths alongside from their anchorage," said the organization.

"These directions are reported to have been issued under threat of violence, with at least one incident of warning shots being fired, and other vessels boarded by armed personnel," it said.

It said the reported incidents "reinforce the threat to vessels visiting Houthi-controlled ports that they are likely to be held or restricted from safely departing."

The United States has intensified airstrikes against Houthi-controlled areas in Yemen since mid-March. In response, the Houthi group launched multiple attacks on the U.S. aircraft carriers and other U.S. warships in the Red Sea.

On April 17, the U.S. army launched a series of attacks targeting and destroying Ras Isa port and concrete tanks storing imported fuel, killing at least 80 people and injuring some 150 others.