(JTA) — The United States flew all non-emergency government staff out of Yemen and called on U.S. citizens to leave the country due to a terrorist threat.

Britain also evacuated its diplomatic staff on Tuesday from the Yemeni capital, Sana. Britain, France, Germany and Norway have closed their embassies in Yemen through the end of Ramadan, the holy month in Islam, later this week.

The actions came several hours after four men believed to be al-Qaida members were killed in Yemen in what is being called an American drone strike.

The U.S. State Department in a message warned American citizens of the “high security threat level in Yemen due to terrorist activities and civil unrest,” and called on Americans “to defer travel to Yemen and those U.S. citizens currently living in Yemen to depart immediately.”

The warnings come after the United States said Monday that the head of al-Qaida had ordered the group in Yemen to attack a U.S. or Western target.

The United States closed its diplomatic missions in Israel and most other Middle East countries on Aug. 4 due to what it called “an abundance of caution.” It extended the closure of several of the embassies through the end of the week.

“This is the most serious threat that I’ve seen in the last several years,” Sen. Saxby Chambliss (R-Ga.), the top Republican on the Senate Intelligence Committee, said Sunday in an interview on NBC’s “Meet the Press.”