Police forces have been warned to be on alert for terrorist attacks after the killing of Al Qaeda leader Ayman Al Zawahiri.
His death in a drone strike in Kabul at the weekend has led to fears that attacks could take place in retaliation.
Former UN terrorism expert Hans Jacob Schindler, director of the Counter Extremism Project, issued a warning to nations to remain vigilant.
“Zawahiri’s death is a significant counter-terrorism success, but it should not be cause for anyone to let their guard down,” he said.
“He is most likely to be succeeded by Saif Al Adel, a revered figure within the terrorist organisation and potentially a bolder operative than Zawahiri.
“Zawahiri was killed in Kabul, which is fully under the control of the Taliban, in a house owned by the acting minister of interior. This demonstrates how close the relationship between the Taliban and Al Qaeda has remained, despite Taliban assurances to the contrary.
“The next Al Qaeda leader is therefore in a good position to organise a base of operations there. It is not at all unlikely that we will see terrorists training in Afghanistan and working toward large-scale, spectacular terrorist attacks.”
On Tuesday, the US State Department warned Americans travelling abroad of potential violence after US forces killed the Al Qaeda chief.
It urged US citizens to “maintain a high level of vigilance and practice good situational awareness when travelling abroad”.
“Current information suggests that terrorist organisations continue to plan attacks against US interests in multiple regions across the globe,” the department said.
“These attacks may employ a wide variety of tactics, including suicide operations, assassinations, kidnappings, hijackings and bombings.”
In a Worldwide Caution Update, the department said it believed “there is a higher potential for anti-American violence given the death of Zawahiri”.
A senior official in the Biden administration said the Egyptian, 71, was on the balcony of a three-storey house in the Afghan capital when two Hellfire missiles struck shortly after dawn Sunday.
It was the first known strike by the US on a target in Afghanistan since Washington withdrew its forces from the country on August 31 last year, days after the Taliban swept back to power.
The US operation involved months of intelligence work to track Al Zawahiri's family to Kabul and identify the target, National Security Council spokesman John Kirby said.
“This mission really took shape over the course of the last six, seven months,” Mr Kirby told CNN. “It was … early this year, as you heard the president say, that we got indications that Zawahiri had moved into Afghanistan.”
Al Zawahiri took control of Al Qaeda after Osama bin Laden, the terrorist organisation’s founder, was killed by American forces in Pakistan in 2011.
President Joe Biden described him as an instrumental player in the planning of the 9/11 attacks on the US and the “mastermind” behind several assaults against Americans, including the 1998 bombings of US embassies in Kenya and Tanzania, and the US Navy destroyer USS Cole in 2000.
Daniel Hoffman, a former senior CIA covert operations officer, said the presence of Al Zawahiri and other Al Qaeda militants in Afghanistan should be “ringing alarm bells”.
He said: “Afghanistan is a clear and present danger. And it has never been more dangerous to the United States of America, I'm sorry to say, than it is right now.”
Before last year's withdrawal, US military leaders said groups such as Al Qaeda could pose a threat from Afghanistan to the US and American allies by 2023.
A UN report last year said as many as 500 Al Qaeda fighters were in Afghanistan and that the Taliban maintained a close relationship with the extremists.