Al-Qaida chief Ayman al-Zawahiri reportedly ordered the militant outfit's deputy based in Yemen to target western diplomatic missions, prompting a swathe of embassy closures across the Middle East and North Africa.
This is believed to have come to light when conversations between the two were intercepted by US undercover agents.
According to top US intelligence officials who spoke on condition of anonymity to the Associated Press and the New York Times, the unusual exchanges between senior al-Qaida figures raised serious suspicions.
Zawahiri is believed to have ordered Nasser al-Wuhayshi, the leader of the al-Qaida affiliate in Yemen, to carry out attacks on western diplomatic missions, in what has been described as one of the most serious plots in recent years. US lawmakers have been describing the threat as strategically significant.
The Yemen-based al-Qaida in the Arabian Peninsula (AQAP) has been growing as a strong offshoot of the militant organisation posing a huge threat to western interests. The AQAP has been responsible for several high-profile attacks in recent years. Al-Wuhayshi is believed to have been one of the closest deputies of slain al-Qaida chief Osama bin Laden.
"This was significant because it was the big guys talking, and talking about very specific timing for an attack or attacks," an American intelligence official who is familiar with the matter told the New York Times.
White House spokesperson Jay Carney said: "Our current information suggests that al-Qaida and affiliated organisations continue to plan terrorist attacks both in the region and beyond. And our information suggests that they may focus efforts to conduct attacks in the period between now and the end of August," he said.
Yemeni intelligence officials have also corroborated the threat. The interior ministry has released the names of 25 senior figures of al-Qaida who were reportedly masterminding an attack on the capital, Sana'a, and other Middle East cities.
Along with the closure of the missions, the US is said to be pressing ahead with its drone attacks, eliminating several al-Qaida militants, local reports say.