An al-Qaeda plot to carry out co-ordinated attacks in the UK, France and Germany has been uncovered, Western intelligence sources say.
Small teams of militants were to seize and kill hostages, similar to the 2008 attacks in Mumbai, the sources said.
Recent US drone raids in Pakistan reportedly targeted the al-Qaeda militants who inspired the plans.
UK officials say the plot has not been stopped, but an attack is not expected to be carried out imminently.
A senior administration official said US President Barack Obama had held "multiple sessions with his CT [counter-terrorism] and homeland teams in recent weeks to review this and other threat reporting and to make sure that all appropriate steps were being taken to protect the American people".
BBC security correspondent Frank Gardner says the plot is believed to have moved from the aspirational stage to actual planning.
Western security agencies may have been hoping to keep the matter out of the public realm for longer so criminal evidence could be gathered, our correspondent adds, but initial details were leaked to the US media.
In the UK, the national threat remains at severe, where it has been since January, meaning a terrorist attack is thought highly likely.
But government officials say there are no plans to raise it to the highest level of critical, and they do not expect to see an imminent wave of arrests.
In an effort to foil the attacks, the US Central Intelligence Agency (CIA) has ramped up missile strikes from unmanned drones against militants in the Pakistani tribal regions, the Wall Street Journal reported, citing security officials.
The US has carried out at least 20 drone strikes so far this month in Pakistan's tribal areas - the highest monthly total for the past six years, US media reported.
Earlier this week, reports from the area said a senior al-Qaeda leader - Sheikh Fate al-Masri - had been killed in a drone attack in North Waziristan.
Security expert and former CIA officer Robert Baer told the BBC's World Today programme he believed the the latest threat to the West may be linked to the US-led attacks on Haqqani insurgents - allies of the Taliban in Waziristan.
"I think what we are facing here is a reprisal from the Haqqani network against the United States and Britain for the stepped-up aerial campaign in the tribal areas of Pakistan," he said.
"You have to look at the way these people look at the world. It is very tribal. They think they are in a feud with the West. They don't understand why they are under attack and they intend to take revenge."
Last week, the US Secretary for Homeland Security Janet Napolitano warned there was "increased activity by a more diverse set of groups and a more diverse set of threats... directed at the West generally".
She is due to meet her European counterparts at a UN aviation security meeting in Montreal this week.
France and Germany are both on a heightened state of security alert.
Germany said on Wednesday that it was aware of a "long-term" aim by al-Qaeda to attack Western targets, but it had no evidence of any "concrete" plans.
It said its security alert level was unchanged.
In the 2008 attacks in Mumbai, 10 gunmen went on a three-day rampage, killing 166 people and injuring more than 300.