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Intercepted conversations between two senior al-Qaeda figures prompted Sunday's closure of many American diplomatic missions, US media report.
The talk, involving top leader Ayman al-Zawahiri, represented one of the most serious plots since the 9/11 attacks, the New York Times says.
The US earlier said the closures in North Africa and the Middle East were "out of an abundance of caution".
Some 20 US embassies and consulates were shut on Sunday.
A state department global travel alert, issued last week, is also in force until the end of August.
US diplomatic posts in Abu Dhabi, Amman, Cairo, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Sanaa and Tripoli will remain closed until Saturday.
Several European countries have also temporarily shut missions in Yemen and the UK Foreign Office is advising against all travel to the country.
Frank GardnerBBC security correspondent
Western governments are taking seriously the perceived threat from al-Qaeda in the Arabian Peninsula (AQAP), because the group has an established track record of highly innovative bomb plots. The bulk of its members are Yemeni militants with a local, Yemeni agenda, but the group has also attracted experienced al-Qaeda operatives from Saudi Arabia with more regional and international ambitions.
These include the elusive Saudi bomb-maker Ibrahim al-Asiri, who sent his brother to blow up a prince with a bomb concealed on, or possibly even inside, his body. Further plots from Yemen have followed, all thwarted, including the Nigerian Omar Farouk Abdulmutallab, who tried to detonate a bomb in his underpants on a flight to Detroit in 2009.
AQAP's leaders have been intensively targeted by US drone strikes in recent years, but by hiding out in remote tribal areas, they have managed to remain largely beyond the reach of the authorities and to continue to plot attacks.
At press briefings, both the White House and the US state department said the threat came from al-Qaeda in the Arabian Peninsula (AQAP), but refused to divulge further details, reports the BBC's Rajini Vaidyanathan, in Washington.
But according to the New York Times, the US intercepted communications between Zawahiri and the group's head in Yemen, Nasser al-Wuhayshi.
The paper said that no targets had been singled out in the discussions, but that a possible attack appeared to be imminent.
A US official told the Associated Press news agency that Zawahiri's message had been picked up several weeks ago and appeared initially targeted at Yemen.
US lawmakers have said it was a huge plot in the final stages, but have offered no specifics.
On Monday, a top member of the House intelligence committee Dutch Ruppersberger told CNN the warnings were not designed to frighten Americans, though he said a planned attack could be "anywhere".
"Americans should live their lives... we just want them to be aware," Representative Ruppersberger said.
Meanwhile, officials in Yemen have released the names of 25 al-Qaeda suspects, saying they had been planning attacks targeting "foreign offices and organisations and Yemeni installations" in the capital of Sanaa and other cities across the country.
There was also increased security at government buildings and checkpoints in Yemen on Monday.
AQAP, the Yemeni branch of al-Qaeda, has also been blamed for the foiled Christmas Day 2009 effort to bomb an airliner over Detroit and explosives-laden parcels that were intercepted the following year aboard cargo flights.
Seven suspected al-Qaeda militants were killed in two US drone air strikes in southern Yemen in June, officials say.