(CNN) -- A pair of suspected U.S. drone strikes killed four al Qaeda militants in Yemen as the United States maintained a heightened security alert in the country and urged all Americans to leave immediately.

Security sources told CNN about the strikes but didn't offer additional details. A Yemeni official said four drone strikes have been carried out in the past 10 days.

None of those killed on Tuesday were among the 25 names on the country's most-wanted list, security officials said.

It is unclear whether the strikes were related to the added security alert in the country after U.S. officials intercepted a message from al Qaeda leader Ayman al-Zawahiri to operatives in Yemen telling them to "do something." The message was sent to Nasir al-Wuhayshi, the leader of al Qaeda in the Arabian Peninsula, the terror group's Yemeni affiliate. U.S. intelligence believes al-Wuhayshi has recently been appointed the overall terror organization's No. 2 leader.

Also Tuesday, the State Department urged Americans in Yemen to leave immediately, citing terrorist activities and civil unrest. All non-emergency U.S. government personnel were also told to leave.

Two U.S. military transport aircraft landed in Yemen on Tuesday to evacuate American citizens.

"In response to a request from the U.S. State Department, early this morning the U.S. Air Force transported personnel out of Sana'a, Yemen, as part of a reduction in emergency personnel," Pentagon press secretary George Little said in a statement.

Little also said, "The U.S. Department of Defense continues to have personnel on the ground in Yemen to support the U.S. State Department and monitor the security situation."

The UK Foreign Office also announced it had temporarily withdrawn all staff from the British embassy and would keep the facility shut until employees are able to return.

Washington takes precautions

Acting on the intelligence information, the United States heightened its security stance, issuing a worldwide travel alert and closing a number of embassies and consulates over large areas of the Middle East and Africa this week.

The State Department said the substantial security steps reflect an "abundance of caution" over intelligence information that indicated final planning by al Qaeda in Yemen for possible terrorist attacks on Western targets to coincide with the end of Ramadan this week.

Three sources told CNN that the United States has information that members of al Qaeda in the Arabian Peninsula are in the final stages of planning for an unspecified attack. Recent jailbreaks in Pakistan, Iraq and Libya all have the fingerprints of al Qaeda operations.

On Monday, White House spokesman Jay Carney told reporters that U.S. anti-terrorism efforts had decimated al Qaeda's global leadership and greatly diminished its core in Afghanistan and Pakistan, saying the threat had "shifted to some of these affiliates, in particular AQAP."

Separately, American special forces units overseas have been on alert for the past several days awaiting a mission to attack potential al Qaeda targets if those behind the most recent terror threats against U.S. interests can be identified, a senior Obama administration official told CNN.

The official declined to identify the units or their locations because of the sensitive nature of the information. Defense Secretary Chuck Hagel put the units on alert last week, the official said.

CNN's Barbara Starr and Hakim Almasmari contributed to this report