NEW YORK: US intelligence intercepted conversations between Pakistan-based Ayman Al Zawahri, who succeeded Osama bin Laden as the head of Al Qaeda, and Nasser al-Wuhayshi, the head of the Yemen-based Al Qaeda in the Arabian Peninsula, which revealed a most serious plot since 9/11 and it forced the Obama administration to shut down some 21 or more US missions in the Middle East, the New York Times reported in its online edition on Monday afternoon.

The NYT said the conversations revealed one of the most serious plots against American and other western interests since the attacks on Sept 11, 2001, citing American intelligence officials and lawmakers.

It is highly unusual for senior Al Qaeda leaders in Pakistan to discuss operational matters with the group’s affiliates, so when the intercepts were collected and analysed last week, senior officials at the CIA, 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 Al 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 told NYT.

In an article posted on the Web on Friday and published on Saturday, the identities of the Al Qaeda leaders whose conversations were intercepted were withheld by The New York Times at the request of senior American intelligence officials. The names were disclosed on 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 on 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 Al Qaeda threat that shuttered many other diplomatic missions emanated in part from that country.

Still, rumours 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.