The US government warned Americans Sunday against traveling to Yemen and authorized the voluntary departure from the country of family members and non-essential embassy staff.
"The Department urges US citizens not to travel to Yemen," the State Department said in a travel advisory.
"US citizens currently in Yemen should consider departing Yemen," the advisory added.
The warning came after suspected Al-Qaeda gunmen killed four soldiers from Yemen's elite Republican Guard in an attack east of the capital Sanaa earlier Sunday.
Unidentified gunmen opened fire on the soldiers as they passed in a truck near Marib, about 170 kilometres (110 miles) east of Sanaa, a Yemeni official said.
On February 22, five people, including three soldiers, were killed in a gunfight with Al-Qaeda militants in Marib, the defence ministry said.
In separate attacks in January, suspected Al-Qaeda militants killed 12 soldiers in ambushes on military convoys and an attack on a military checkpoint in the south of the country.
A US State Department official last month described Yemen-based Al-Qaeda in the Arabian Peninsula -- a fusion of the Yemeni and Saudi branches of the jihadist network -- as the "most significant" threat to the US homeland.
The advisory said that the security threat level in Yemen was "extremely high due to terrorist activities and civil unrest."
Piracy in the Red Sea, Gulf of Aden and Indian Ocean is also a security threat to maritime activities in the region, the State Department added.
"Should a crisis occur, evacuation options from Yemen would be extremely limited due to the lack of infrastructure, geographic constraints, and other security concerns," said the State Department. "The US Embassy's ability to assist US citizens in the event of a crisis in Yemen is very limited."