SANAA - Six al Qaeda militants were killed in an air strike in northern Yemen on Friday in a stepped-up campaign by the Yemeni government against the Islamist militant group.

Yemen, which gained a reputation as a haven for al Qaeda after the September 11, 2001 attacks in the United States, came under the spotlight after crackdowns on al Qaeda in Pakistan and Afghanistan raised concern that it was becoming a training and recruiting center for militants.

Two cars carrying eight dangerous al Qaeda members were hit in an area between Saada and al-Jouf, a Yemeni security official told Reuters. Two may have survived and escaped.

The group included Qassem al-Remi, Ayed al-Shabwani, Ammar al-Waeli, and Saleh al-Teys, the official said, adding that the four were wanted by the Yemeni and U.S. security services.

It is believed that Qassem al-Remi and Ayed al-Shabwani were killed in the air strike.

Al-Shabwani was one of the most dangerous al Qaeda members who provided a hideout for other militants at his farm in Maarib, where their training took place, the official said.

Maarib is a mountainous eastern province where the oil and gas fields of major international companies are located.

Yemen had already stepped its operations against al Qaeda since a Yemen-based wing of the group said it was behind an attempt to blow up a Detroit-bound U.S. airliner on December 25 .

The foiled bombing has focused attention on the growing prominence of al Qaeda in Yemen and the expanding role of the U.S. military and intelligence agencies in fighting it.

Yemen declared open war on al Qaeda on Thursday and warned its citizens against aiding the global militant group. Authorities sent troops this week to join a drive against al Qaeda in three provinces.

The poor Arab country, home to 23 million people, has come under pressure to act against al Qaeda since attacks on its two main allies, Saudi Arabia and the United States, by militants from Yemeni soil.

Besides fighting al Qaeda in several provinces, Yemen is facing a Shi'ite revolt in the north and a separatist drive in the south.

Authorities launched an operation this week to root out al Qaeda militants who they said were behind militant threats that forced Western embassies to close on Sunday.

Yemen has long been a base for al Qaeda. Islamic militants bombed the warship USS Cole in the Yemeni port of Aden in 2000, killing 17 U.S. sailors.

Yemenis were one of the largest groups of militants to train in al Qaeda's camps in Afghanistan before the September 11 attacks.

The West and Saudi Arabia fear al Qaeda will exploit Yemen's instability to spread its operations to its neighbor, the world's biggest oil exporter, and beyond.

(Writing by Samia Nakhoul; editing by Andrew Dobbie)