Masked gunmen shot dead a Yemeni man on his way to work at the U.S. embassy in Sanaa on Thursday, a security source said, the latest in a wave of assassinations in the Arab state where Washington is battling al Qaeda militants.The attackers on a motorcycle opened fire on a car carrying Qassem Aqlan - who headed an embassy security team - in the centre of Yemen’s capital, the source told Reuters."This (assassination) operation has the fingerprints of al Qaeda which carried out similar operations before,” said the source who asked not to be named.The Yemen-based al Qaeda in the Arabian Peninsula and other militant groups strengthened their grip on parts of the country during an uprising that ousted veteran President Ali Abdullah Saleh in February.There have been a number of assassinations attempts, some of them successful, on security officials and politicians since Yemen’s army drove Islamist fighters out of several southern towns earlier this year.Washington, wary of the growing power of al Qaeda, has stepped up drone strikes on suspected militant positions, with the backing of Saleh’s successor, President Abd-Rabbu Mansour Hadi.Men armed with machine guns and rockets attacked a security checkpoint in Yemen’s southern city of al-Dalea late on Wednesday, injuring two policemen, a local official said on Thursday.The attackers, whose affiliation was not immediately clear, fled the scene, the official said.