SANAA, Yemen, (AP) – Suspected al-Qaeda gunmen killed four Republican Guard soldiers on Sunday in the mountainous central province of Marib, security officials said.

Also Sunday, the U.S. government, citing terrorism and civil unrest, advised Americans not to travel to Yemen and said Americans already in the impoverished Arab nation should leave. A travel warning issued by the State Department also authorized family members of U.S. Embassy staff and non-essential personnel to leave.

The security officials said the suspected al-Qaeda gunmen ambushed the soldiers while traveling in a vehicle to distribute food to fellow troops manning checkpoints. They said the assailants managed to flee.

Al-Qaeda militants have frequently attacked army soldiers in Marib province, an al-Qaeda stronghold and one of several Yemeni provinces where the authority of the central government is weak.

The officials spoke on condition of anonymity because they are not authorized to brief the media.

Al-Qaeda’s activity has grown in Yemen over the past year, with several foiled terror attacks in the United States and elsewhere traced back to operatives based there.

The country has also been thrown into turmoil in recent weeks, with daily demonstrations across the nation by protesters demanding that longtime president Ali Abdullah Saleh step down. Saleh has declared he would not seek another term in office in 2013, but his concession failed to appease the protesters.