The intercepted conversations last week between Ayman al-Zawahri, who succeeded Osama bin Laden as the head of the global terrorist group, and Nasser al-Wuhayshi, the head of the Yemen-based Al Qaeda in the Arabian Peninsula, revealed one of the most serious plots against American and other Western interests since the attacks on Sept. 11, 2001, American intelligence officials and lawmakers have said.
It is highly unusual for senior Qaeda leaders in Pakistan to discuss operational matters with the group's affiliates, so when the intercepts between the two senior Qaeda leaders were collected and analyzed last week, senior officials at the C.I.A., the State Department and the White House immediately seized on their significance. Members of Congress were quickly provided classified briefings on the matter, American officials said.
"This was significant because it was the big guys talking, and talking about very specific timing for an attack or attacks," said one American official who had been briefed on the intelligence reports in recent days.
The identities of the two Qaeda leaders whose discussions were monitored and the imminent nature of the suspected plot — in the intercepts, the terrorists mentioned Sunday as the day that the attacks were to take place — help explain why the United States, as well as other Western governments, have taken such extraordinary precautionary steps in the past few days to close embassies and consulates in the Middle East and North Africa.
But the intercepts were frustrating in that they did not reveal the specific location or target of the attacks, American officials said.
In an article posted on the Web on Friday and published on Saturday, the identities of the Qaeda leaders whose conversations were intercepted were witheld by The New York Times at the request of senior American intelligence officials. The names were disclosed Sunday by McClatchy Newspapers, and after the government became aware of the article, it dropped its objections to The Times's publishing the same information.
The State Department said Sunday that it was extending the closing of 19 diplomatic posts in the Middle East and North Africa through at least next Saturday because of continued fears of an imminent attack. Several European countries have also closed embassies in the Middle East.
A State Department spokesman said that the closings were not the result of new threat intelligence, but "merely an indication of our commitment to exercise caution and take appropriate steps to protect our employees and visitors to our facilities."
The embassies that will be closed include the ones in Yemen, Libya, Egypt, Saudi Arabia and the United Arab Emirates, the statement said.
The American Embassy in Pakistan has remained open, even though the Qaeda threat that shuttered many other diplomatic missions emanated in part from that country. Still, rumors of an impending militant attack on Islamabad, the capital — and not necessarily on an American target — coursed through diplomatic and security circles last weekend.
One Western diplomat, speaking on the condition of anonymity, said his mission had received reports that militant attackers had congregated in the Margalla Hills, which overlook the city. But the diplomat stressed that those reports were unconfirmed, and that while the security situation in the city had tightened, there was little information to suggest an impending assault.
Administration officials and intelligence analysts said on Monday they had no new information on the terrorist threats.
"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," said a State Department spokeswoman, Marie Harf. "Over all, what we are doing is taking precautionary steps out of an abundance of caution to protect our people and our facilities and visitors to those facilities overseas."
Members of Congress who were briefed on the threat said there was no definitive information on where an attack would occur.
"The assumption is that it's probably most likely to happen in the Middle East," Representative Peter T. King, Republican of New York said on Sunday on the ABC News program "This Week." "But there's no guarantee of that at all."
"It could basically be in Europe, it could be in the United States, it could be a series of combined attacks," said Mr. King, who is a member of the House Intelligence Committee.
The one aspect of the intelligence that officials appear to agree on is that Al Qaeda's affiliate in Yemen is behind the plotting.
The group, Al Qaeda in the Arabian Peninsula, has tried to carry out several high-profile attacks in recent years. One was an attempt to blow up a trans-Atlantic jet over Detroit on Dec. 25, 2009, using explosives sewn into a passenger's underwear. Months earlier, the group tried to kill the Saudi intelligence chief with a bomb surgically implanted in the attacker's body.
American officials believe that both bombs were built by Ibrahim al-Asiri, one of the group's leaders, whom the Obama administration has been trying to kill as part of a campaign in Yemen using armed drones.
Declan Walsh contributed reporting from London.
