C AIRO — Iran on Sunday once again denied aiding Yemen's Houthi rebels after the United States launched a wave of airstrikes against them and President Donald Trump warned that Tehran would be held “fully accountable” for their actions.

The U.S. carried out new airstrikes on Yemen, expanding the biggest U.S. military operation in the Middle East since President Trump took office.

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.