SANAA (Reuters) - The United States told its citizens on Sunday to avoid traveling to Yemen and said those already there should consider leaving due to deteriorating security.
The warning was issued as protests aimed at ousting President Ali Abdullah Saleh, an ally of Washington against a resurgent Yemen-based Al Qaeda arm, swelled in the Arabian Peninsula state, sometimes into the hundreds of thousands.
"The Department (of State) urges U.S. citizens not to travel to Yemen. U.S. citizens currently in Yemen should consider departing," the U.S. State Department said in a travel warning.
"The security threat level in Yemen is extremely high due to terrorist activities and civil unrest."
Suspected al Qaeda militants killed four Yemeni police officers in an ambush Maarib province, east of the capital Sanaa, a local official said on Sunday.
The police vehicle had been delivering supplies to forces in the city of Maarib when the attack occurred.
The growing Yemeni protests, and a series of defections from Saleh's allies, have added pressure on Saleh to end his three-decade rule. But the president is sticking to an earlier plan to step down only when his current term ends in 2013.
Yemen, a neighbor of Saudi Arabia, was teetering on the brink of failed statehood even before recent protests, with Saleh struggling to cement a truce with Shi'ite rebels in the north and quell a secessionist rebellion in the south.
"Terrorist organizations continue to be active in Yemen, including Al Qaeda in the Arabian Peninsula. The U.S. government remains concerned about possible attacks against U.S. citizens, facilities, businesses, and perceived U.S. and Western interests," the U.S. statement said.
Political analysts say the recent protests, inspired by unrest that has toppled the leaders of Egypt and Tunisia, may be reaching a point where it will be difficult even for Saleh, a clever politician, to cling to power.
Yemeni protesters say they are frustrated with corruption and soaring unemployment in a country where 40 percent of its 23 million people live on $2 a day or less and a third face chronic hunger.
The U.S. State Department said its ability to help U.S. citizens in a crisis could be restricted, and that evacuation options would be "extremely limited" due to a lack of infrastructure, geography and security concerns.
It authorized the voluntary departure of the family members of U.S. embassy staff and non-essential personnel.
(Reporting by Mohamed Sudam; Writing by Cynthia Johnston)