Britain said Sunday that it has a small diplomatic team in Benghazi but refused to confirm reports that a diplomat and Special Air Service soldiers were being held in Libya's second city.
The Sunday Times newspaper said the SAS unit was captured along with the junior diplomat they were escorting through the rebel-held east and who was seeking contact with opponents of Libyan leader Moamer Kadhafi.
"We can neither confirm nor deny the report," a Foreign Office spokeswoman told AFP.
A correspondent for the BBC based in Benghazi said he had been informed that a helicopter carrying six people dressed in black and carrying weapons had landed in the region in the early hours of Friday.
"These six people who came off the plane were in black clothings which make them sound like they were SAS forces," he told BBC television.
According to his sources, they were arrested and taken to a military base although the situation was expected to be resolved fairly soon.
"The opposition here have an understanding of the situation that these people are not hostile people. The problem was to arriving on a helicopter, in the middle of the night, carrying weapons. You can understand the sort of fear that provoked here," the correspondent added.
The defence ministry said it did not comment on the special forces, while Defence Secretary Liam Fox told BBC television: "I can confirm that a small British diplomatic team is in Benghazi.
"We are in touch with them but it would be inappropriate for me to comment further," he added.
For his part, Mustafa Gheriani, spokesman for the rebels' self-declared national council in Libya's second city of Benghazi, refused to comment.
"I have absolutely nothing to say," he told AFP.
The Sunday Times claimed that the uninvited appearance of the SAS alongside the diplomat "angered Libyan opposition figures who ordered the soldiers to be locked up in a military base".
Opponents of Kadhafi "fear he could use any evidence of Western military interference to rally patriotic support for his regime", the weekly broadsheet added.
The newspaper said that according to Libyan sources, the SAS soldiers were taken by rebels to Benghazi, held by the opposition, and hauled up before a senior figure.
The Sunday Times said a British source, who confirmed the men had been detained, said the diplomat they were protecting had wanted to make contact with the rebels to prepare the way for a visit by a senior colleague.
It cited a source close to the opposition leadership as saying rebel officials were worried that Libyan people might think from the escort party that "foreign troops have started to interfere by landing in Libya".
British service personnel have already been involved in the rescue of British nationals working on oil installations in remote desert camps.
Prime Minister David Cameron last week said Western countries should be stepping up contact with the Libyan opposition to gain a greater understanding of their intentions.
In a statement Sunday, Foreign Secretary William Hague called upon Kadhafi "to put an immediate stop to the use of armed force against the Libyan people".
He added: "The UK reiterates its support for the transition to a government that will deliver greater democracy, justice, transparency, human rights and accountability in Libya."
In Britain on Saturday, the defence ministry said about 200 troops had been placed on standby to help with evacuation and humanitarian operations in Libya.
However a YouGov poll of 2,413 adults for The Sunday Times found low support for using troops in Libya.