The US embassy in Yemen said on Monday it had received a threat of a possible attack and urged Americans to be cautious in the Arab country that has been the scene of al Qaeda attacks on Western interests.
Twin suicide car bombings killed 16 people outside the heavily fortified US embassy in Sanaa in September, in an attack later claimed by al Qaeda.
An embassy employee told Reuters by telephone on Monday that the mission remained open.
"The U.S. Embassy has received a threat against the embassy compound regarding a possible attack which could take place in the foreseeable future," it said on its website.
"U.S. citizens in Yemen are advised to exercise caution and take prudent security measures in all areas frequented by Westerners, including maintaining a high level of vigilance, avoiding crowds and demonstrations, keeping a low profile..." it said in a warden's report.
The statement came days after al Qaeda's wing in Yemen issued a video on the Internet, carrying a call by its leader for attacks on Western targets, and saying two Saudis released from the US military prison camp in Guantanamo Bay had joined the group as commanders.
"We have to cut off the supplies of the Zionist-crusader (Israeli-Western) campaign and kill any of the crusaders we find on our land," Abu Basir al-Wahayshi, leader of al Qaeda in the Arabian Peninsula, said on the video.
After the video was issued, a Saudi Interior Ministry official said nine militants who had undergone "rehabilitation" before being released, including some Guantanamo returnees, had been re-arrested, the state news agency SPA reported. The official did not give the exact dates of the arrests.
Yemeni President Ali Abdullah Saleh said on Saturday he expected the repatriation soon of 94 Yemenis held at the Guantanamo Bay prison camp, adding that they would be living with their families at a centre to undergo "rehabilitation ... to rid them of extremism" before being released.
Yemen, al Qaeda leader Osama bin Laden's ancestral home, joined the US-led "war on terrorism" after the September 11 attacks on US cities in 2001.
Yemen has jailed scores of militants in connection with the bombings of Western targets and clashes with authorities, but is still viewed in the West as a haven for Islamist militants.