An April 15, 2011 video screenshot shows al-Qaeda number two Ayman al-Zawahiri delivering a speech. (AFP Photo / HO / Site Intelligence Group/AFP/Getty Images)

Al-Qaeda leader Ayman al-Zawahiri ordered the head of the terrorist group’s Yemen affiliate to carry out an attack, according to intercepted communications that have led to the closure of U.S. embassies and a global travel alert, said a person briefed on the case.

In one communication, Zawahiri, who succeeded Osama bin Laden, gave “clear orders” to Nasir al-Wuhayshi, the founder of al-Qaeda in the Arabian Peninsula, to undertake an attack, the source said. McClatchy newspapers first reported the exchange on Sunday.

Concern about a terror plot has prompted the United States to shutter embassies and consulates across the Middle East and North Africa at least through Saturday. On Tuesday, the U.S. Embassy in Sana’a, Yemen urged all U.S. citizens inside to leave the country immediately, and announced that non-essential embassy personnel were being evacuated “due to the continued potential for terrorist attacks.”

Wuhayshi, who was once bin Laden’s personal secretary, recently was elevated by Zawahiri to the number two position in al-Qaeda. That is a sign, analysts said, that Zawahiri is seeking to compensate for the weakness of al-Qaeda’s core group in Pakistan by working more through regional affiliates.

“It’s very worrisome because al-Qaeda in the Arabian Peninsula is the most effective and threatening affiliate,” said Bruce Hoffman, a terrorism expert and Georgetown University professor. “So now the leader of the most consequential affiliate has an intimate command role in the overall organization. From Zawahiri’s point of view, there’s no better examplar of the Qaeda brand than AQAP.”

The Post's Anne Gearan explains the Obama administration's decision to close 19 diplomatic posts across the Middle East and North Africa and what it tells terrorist organizations and leaders in the region. (The Washington Post)

The group has been linked to a foiled plot to bomb a passenger jet bound for Detroit on Christmas Day 2009 as well as one in 2010 to blow up cargo planes with bombs hidden in printer cartridges. Wuhayshi is still thought to be in Yemen and Zawahiri in Pakistan, Hoffman said.

U.S. officials had no new comment on the nature of the plot that shuttered embassies and consulates across the Middle East and Africa.

White House press secretary Jay Carney said Monday that the threat “is emanating from and may be directed toward the Arabian Peninsula, but it is beyond that, potentially. And that is why we have taken some of the actions we’ve taken.”

Nineteen embassies and consulates will remain closed at least through Saturday, officials said. Embassies in Afghanistan and Iraq reopened Monday after a closure of one day. Carney said the continued closures of embassies does not reflect new threat information but “is more a reflection of taking necessary precautions.”

“We are going to keep evaluating information as it comes in, keep analyzing the various intelligence that we’re getting in in regards to this stream,” State Department spokeswoman Marie Harf said Monday.

Members of Congress who were briefed on the threat said Sunday that it resembled the “chatter” or streams of intelligence that preceded the Sept. 11, 2001, attacks on the United States. The scope and duration of the embassy closures are “almost without precedent,” Hoffman noted.

The threat comes during a national debate over the proper scope of surveillance programs to detect terrorism plots. Officials cautioned against conflating the issues. “We have a threat that we have advised the public about,” Carney said. “And we have a separate set of issues . . . that has led to a debate about the [surveillance] program.”

One program that allows the collection of domestic phone records has been particularly controversial. But Rep. Adam B. Schiff (D-Calif.), a member of the House Intelligence Committee, said he would be “very surprised” if that program contributed to the uncovering of a “foreign-based plot involving primary targets like embassies and consulates.”