Security forces in Yemen were on a heightened state of alert Wednesday, a day after the United States urged all Americans to leave the country amid fears of a possible terror threat.

Two U.S. military transport aircraft landed in Yemen on Tuesday to evacuate American citizens. The UK Foreign Office has also withdrawn its embassy staff.

A senior Yemeni Interior Ministry official told CNN that "a few" al Qaeda operatives have arrived in the Yemeni capital, Sanaa, over the past three days, which has forced government authorities to put Yemeni forces on high alert. The official could not be named, as he is not authorized to speak to the media.

"We are confident that government security forces will be able to stop any attack from taking place in Sanaa, but the militants do have a good history of operating in Sanaa," he said Tuesday.

"It's not a secret that al Qaeda also has sleeper cells in Sanaa," the source said.

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

U.S. State Department spokeswoman Jen Psaki said Tuesday that the United States has "a close partnership" with Yemen and that Secretary of State John Kerry had spoken with President Abdu Rabu Mansour Hadi to thank him for his efforts.

But in a sign of possible tensions with its international allies, Yemen's Ministry of Foreign Affairs said the evacuation of embassies "serves the interests of the extremists" and undermined its efforts.

"Yemen has taken all necessary precautions to ensure the safety and security of foreign missions in the capital Sanaa," a statement said Tuesday.

"While the government of Yemen appreciates foreign governments' concern for the safety of their citizens, the evacuation of embassy staff serves the interests of the extremists and undermines the exceptional cooperation between Yemen and the international alliance against terrorism."

Helicopter shot down

Two separate drone strikes on two vehicles killed six al Qaeda militants in the southern province of Shabwa on Wednesday morning, officials said.

The latest strikes came a day after a pair of suspected U.S. drone strikes killed four al Qaeda militants in Yemen. Security sources told CNN about those strikes but didn't offer additional details. None of those killed Tuesday were among the 25 names on the country's most-wanted list, security officials said.

It was unclear whether Tuesday's strikes were related to the security alert in place in the country since 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.

Meanwhile, a Yemeni government official not authorized to speak to media told CNN that a military helicopter was shot down in Mareb province Tuesday.

He said that the helicopter had been inspecting the country's main oil pipeline -- one that has been subject to repeated attacks -- and that "it's possible this is the work of AQAP."

At least eight people were killed, including the 107th Brigade commander, six army escorts and at least one crew member, he said.

AQAP's recent attacks have included a suicide bombing on a pro-government militia in the south in March that killed 12, and an attempted suicide bombing attack on a gas pumping facility in the port city of Balhaf in June.

In July, several soldiers were killed by a bomb in Sanaa after a lull in attacks in the capital.

American citizens evacuated

The State Department on Tuesday urged Americans in Yemen to leave immediately, citing terrorist activities and civil unrest. All nonemergency U.S. government personnel were also told to leave.

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

"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," he added.

Psaki said those evacuated had been taken to Germany.

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

'Abundance of caution'

The State Department said the substantial security steps it has taken 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.

The Yemeni security forces, extensively reorganized under Hadi, have over the past 18 months recaptured swathes of territory that were briefly held by al Qaeda in the Arabian Peninsula, particularly in the south of the country.

AQAP has not mounted a large-scale suicide attack on Yemen's security forces since May 2012, when more than 100 soldiers were killed by a suicide bomber as they trained for a parade in Sanaa.

Many of AQAP's operatives, including its leadership, retreated into remote areas after the Yemeni military offensive last year and regrouped.

CNN's Mohammed Jamjoom reported from Beirut and journalist Hakim Almasmari from Sanaa, while Laura Smith-Spark wrote in London. CNN's Elise Labott, Barbara Starr, Tim Lister and Paul Cruickshank contributed to this report.