By Mohammed Ghobari and Yara Bayoumy
SANAA, Jan 26 (Reuters) - A suspected U.S. drone strike on a car in eastern Yemen killed three men believed to be al Qaeda militants on Monday, officials said, suggesting Washington's remotely-piloted air war has survived the overthrow of the country's U.S.-backed leader.
President Abd-Rabbu Mansour Hadi and his government quit on Thursday after he was besieged inside his palace by an Iranian-allied armed group, depriving Washington of an ally in its campaign against al Qaeda in the Arabian Peninsula (AQAP), one of the most powerful branches of the global Jihadist network.
The attack in Marib province was the first apparent drone strike since the departure of Hadi, and signals Washington's campaign against AQAP is continuing, at least in its use of drones, despite his absence.
AQAP claimed responsibility for deadly Charlie Hebdo shootings in Paris on Jan. 7 and American officials fear al Qaeda will gain strength in Yemen's current power vacuum.
Two U.S. security officials had said on Friday the collapse of Hadi's U.S.-backed government left America's counter-terrorism campaign "paralyzed," but Monday's strike suggests the CIA-run drone campaign has not been dismantled.
President Barack Obama on Sunday defended his drone strategy against AQAP, saying the alternative would be to deploy U.S. troops, which was not sustainable.
The U.S. embassy said on Monday that it was closing to the public until further notice due to security concerns. Officials in Washington had said last week that staff levels at the mission were being reduced.
Shi'ite Muslim Houthi militiamen seized the capital Sanaa in September, becoming Yemen's dominant political faction, and now run the capital and several other parts of the country. Last week they seized the presidential palace and besieged Hadi in his residence in a dispute about the constitution that ended with Hadi's resignation.
The fighters, bedecked in tribal robes and automatic weapons, have set up checkpoints throughout the city.
On Monday, Houthi gunmen and supporters stormed the capital's main university to detain several people from among 200 or so protesters who had demonstrated against their rule.
A group of activists who took part in the 2011 Arab Spring uprising in Yemen, which ousted Hadi's predecessor, veteran autocrat Ali Abdullah Saleh, warned in an open letter on Monday that the Houthi takeover could destroy the Yemeni state and dash their hopes for a transition to democracy.
"This path increases the chances of armed conflict which can only result in a civil war and fragment the country on fanatical sectarian and regional lines, putting an end to the dream of Yemenis for a civil state," the activists wrote.
The Houthis, representing a Shi'ite minority that ruled a thousand-year kingdom in Yemen's northern highlands until 1962, made steady gains southward last year and finally seized Sanaa, aided, diplomats say, by the inaction of army units loyal to Saleh, once their implacable foe.
While the Houthis say their takeover was a second revolution aimed at ousting corrupt officials, some activists fear they are an Iranian proxy working with the former president to settle scores and become new kingmakers.
"Just as in 2011, we are using peaceful means in confronting these militias," said Fayez Noman, a socialist protester near Sanaa University, standing near the center of the old uprising dubbed "Change Square" now sealed off by gunmen.
"We will continue to take to the streets every day. We have no other option."
Fanning out to majority Sunni areas in the country's west and center, the Houthis have met resistance from tribes allied with AQAP, leading to an increase in deadly sectarian combat.
Six Houthi fighters were killed in ambush on their car by local gunmen on Monday, tribal sources said. (Additional reporting and writing by Noah Browning; Editing by Peter Graff)