WASHINGTON, April 14 -- The United States on Tuesday imposed sanctions on the leader of the Houthi rebels in Yemen and a son of former President Ali Abdullah Saleh for allegedly supporting the rebels.

The Department of Treasury targeted Abdul Malik al-Houthi, whose Shiite Houthi movement has overrun much of Yemen in the past few months, including the capital city of Sanaa, and Ahmed Ali Saleh, former head of Yemen's Republican Guard and "a key supporter" of the Houthis.

The agency freezes the assets of the pair under U.S. jurisdiction and bars Americans from doing business with them, in a move that came after the UN Security Council passed a resolution earlier in the day hitting them with an asset freeze and a travel ban.

"Using violence and other means, al-Houthi and Saleh continue to undermine the political process in Yemen and obstruct the peaceful transition of power in Yemen despite repeated calls from the international community that they desist from their destabilizing actions," the Treasury said in a statement.

The Obama administration has expedited the delivery of weapons in support of Saudi-led airstrikes on the rebels that started on March 26, on top of intelligence sharing and daily aerial refueling.

Washington stepped up efforts to help Yemen fight al-Qaida in the Arabian Peninsula after Abd-Rabbu Mansour Hadi assumed the presidency following Saleh's ouster in November 2011.

The U.S. had to withdraw its diplomats and some 100 troops from the country last month in the face of the rebels' advances, which also forced Hadi into exile in neighboring Saudi Arabia.

Military units loyal to Ali Abdullah Saleh and his son are accused of involvement in the rebels' offensives.