SANAA - Yemeni forces fought al Qaeda militants on Monday, killing at least two they said were behind security threats which forced the U.S. embassy and other foreign missions to close.

These (militant) elements are believed to be behind the threats directed to the U.S. embassy, a Yemeni security official told Reuters. Clashes are still continuing.

The United States embassy in the capital, Sanaa, said it was staying shut for a second day in response to al Qaeda threats.

Britain's embassy has been closed since Sunday and Japan suspended its consular services on Monday due to security concerns.

Al Qaeda in the Arabian Peninsula, the Yemen branch of Osama bin Laden's network, has claimed responsibility for a December 25 attempt to blow up a U.S. passenger plane carrying 300 people.

It said the Christmas Day bomb attempt was retaliation for U.S. involvement in Yemen and its support for the government's offensive against the militants.

Umar Farouk Abdulmutallab, the 23-year-old Nigerian accused of trying to blow up a plane as it approached Detroit, spent time in Yemen last year where U.S. officials believe he received training from a militant group.

The events have swung the focus on Washington's war on Islamist militants onto Yemen while U.S. forces are fighting a strengthening Taliban insurgency in Afghanistan and are still committed in Iraq.

Both the United States and Saudi Arabia fear al Qaeda may exploit instability in the poorest Arab country to turn it into a launch pad for more attacks.

Yemen has tightened coastal security after Somali militants said they were ready to send reinforcements for al Qaeda.

In addition, Yemen faces a Shi'ite Muslim rebellion in the north and separatist protests in the south.

The conflict with the northern Shi'ite rebels -- who complain of social, economic and religious marginalisation -- has killed hundreds of people and displaced tens of thousands.

The rebels said Saudi Arabian war planes killed 16 Yemenis in air strikes over the last two days in the border area.

Saudi Arabia was drawn into the conflict in November when rebels staged a cross-border incursion into the world's biggest oil exporter.

The Houthi rebels -- named after their leader Abdul-Malik al-Houthi -- said on their website that six people were killed and six more wounded, including women and children, when the strikes destroyed two houses on Sunday.

A day earlier, strikes killed 10 and wounded 13 in a market in another area, the rebels said in a separate statement.

Saudi defence ministry officials could not immediately be reached for comment.
In the mainly Sunni Muslim south, Yemen has sporadically clashed with separatist protesters seeking independence for southern Yemen, which unified with its northern neighbour in 1990 and failed to secede in a 1994 war.

(Writing by Robin Pomeroy; Editing by Angus MacSwan)