ADEN (Reuters) - Two men thought to be Islamist militants were killed in an apparent U.S. drone attack on a car in central Yemen on Tuesday, the defense ministry said.

A security source and witnesses told Reuters the car was hit on a remote road from Hadramout to Maareb province - a mostly desert southeastern region where militants have taken refuge after being driven from their southern strongholds last month.

It was not clear if there were other casualties in the attack. Washington, which fears the spread of militants in Yemen, has stepped up attacks by unmanned drones this year.

Earlier on Tuesday, a presidential adviser said he survived an apparent assassination attempt on Monday when armed men opened fire on a car carrying him in Sanaa.

The impoverished Arabian Peninsula state has been in turmoil since an uprising erupted last year and forced veteran ruler Ali Abdullah Saleh to step down in February.

Islamist militants gained ground during the unrest, which was inspired by revolts across the Arab world, taking control of several towns in south Yemen.

The army, with backing from the United States, has forced them out of some areas this year but they have hit back with a series of suicide bombings targeting government institutions.

Al Qaeda in the Arabian Peninsula is based in Yemen and has mounted operations in neighboring Saudi Arabia and against the United States.

(Reporting By Mohammed Mukhashaf; Writing by Mirna Sleiman; Editing by Andrew Roche)