"Any act of aggression will have severe consequences, for which the U.S. will bear full responsibility," Iranian Permanent Representative to the United Nations Amir Saeid Iravani wrote in a letter to U.

The comments by Trump on his Truth Social website escalate his administration's new campaign of airstrikes targeting the rebels.

Oil prices rose slightly on Monday after the United States vowed to keep attacking Yemen's Houthis until the Iran-aligned group ends its assaults on shipping, while Chinese economic data buoyed hopes for higher demand.

Oil futures climbed on Monday, finding support after the U.S. over the weekend launched U.S. strikes on Houthi rebels in Yemen, raising the risk of disruptions to oil flow in the region.

Oil prices were up nearly 1 per cent on Monday after the US bombed Yemen's Houthis for a second night, warning of an “unrelenting” attack on the militant group, and as China announced stimulus plans to boost its tepid economy.