UK forces have arrested two Britons in Afghanistan amid fears they may have been planning an attack on British troops in the country.
The pair were held at a hotel in the western city of Herat in a dramatic joint raid with Afghan intelligence service the National Directorate of Security (NDS), The Times reported.
The suspects, who are British passport-holders with dual nationality, are now being held by UK forces in Helmand Province in southern Afghanistan, according to the paper.
An unnamed senior Afghan official told The Times: "It was terrorism-related. It was a UK-led operation."
The Ministry of Defence and Foreign Office confirmed the arrests but would not give any further details about the operation.
An MoD spokesman said: "We can confirm that British forces have detained two individuals in Afghanistan who claim to be British nationals. We are not prepared to comment further at this stage."
A Foreign Office spokeswoman added: "We can confirm that two British nationals have been detained in Afghanistan. Embassy staff are providing assistance."
Nato forces in Afghanistan normally hold suspects for a maximum of four days before releasing them or handing them over to the Afghan authorities, but this can be extended in certain cases.
The Foreign Office spokeswoman said: "The UK has a national policy of detaining beyond 96 hours in exceptional circumstances, in particular where it could provide information that could help protect our forces or the local population."
News of the arrests came as the UK handed responsibility for security in Helmand's capital, Lashkar Gah, to Afghan forces yesterday in a symbolic step towards the planned pull-out of British combat troops by the end of 2014.
It is understood there is no link between the arrests and any potential terror plot in the UK.
Legal charity Reprieve, which campaigns for prisoners' rights, has written to the Foreign Office warning that the suspects could face torture or the death penalty if they are transferred to Afghan custody.
Director Clive Stafford-Smith asked British officials to confirm that the men had been permitted access to lawyers and to offer them his group's assistance.
Reprieve said it had not received a response from the Foreign Office so far.
A Ministry of Defence spokesman said: "We can confirm that two British passport holders were detained in an Isaf operation conducted jointly last week by UK forces with Afghans in support. The individuals, a male and a female, are currently being held in a secure facility in Kandahar for questioning.
"Detention operations are a vital element of protecting UK, Isaf and Afghan forces and Afghan civilians from those who are assessed to pose a threat regardless of their nationality.
"All detention operations in Afghanistan carried out under the remit of the UN-mandated International Security Assistance Force (Isaf), which includes those conducted by British forces, are conducted in accordance with international law and strict policy frameworks."