Reporter Nicholas Davies and cameraman Gareth Montgomery-Johnson were detained by members of the Misrata Brigade on Tuesday and are being held in Tripoli.
It is not clear if Mr Montgomery-Johnson, a cameraman from Carmarthen, and his colleague will face charges.
Human Rights Watch says it has been denied access to the journalists.
The two men were working for the Iranian-owned, English language television station, Press TV, and were apparently filming in the capital when they were detained by militiamen on Tuesday, alongside two Libyan colleagues.
The British Embassy in Tripoli told the BBC it was providing consular assistance and the men appeared in good health.
However, after more than five days in detention, it is not clear when the men might be transferred into official custody or when any charges might be brought.
When contacted by telephone, the commander of the brigade, Faraj Sweihli, told the BBC he would not discuss the case.
Mr Sweihli is understood to have told others that the journalists’ documents were not in order, and that they were uncooperative when they were detained.
Human Rights Watch (HRW) says its representatives have been denied access to the journalists, despite letters of permission from the government and the National Transitional Council.
The militia apparently stopped the men because they were driving late at night in Tripoli and taking photographs, which the militia found suspicious.
The two journalists were then accused of not having the proper immigration papers.
“If these journalists have immigration problems, then the appropriate Libyan authorities should deal with this, and not a self-appointed militia with no legal power to make arrests or hold detainees,” said a spokesman for HRW.
The Libyan government has asked the militia to transfer the two journalists to the government’s custody.
Mr Davies, 37, works under the name Nick Jones.
Two local residents were also seized in the incident.
Mr Montgomery-Johnson’s sister Melanie Gribble told BBC Wales the family were concerned for his welfare.
She said her brother, 36, who is from Carmarthen, had just returned to Libya after a two-week break to relax after working in the country since July last year.
She said she was in regular contact with the Foreign Office and had been told her brother was suffering from an ear infection and had seen a doctor.
She added the men were “getting on well with their guards”, but that “they are very tired”.
The case highlights the tensions between the interim authorities and the various armed groups.
Most of these militias say they are loyal to the government.
In reality, the politicians are often powerless to control the bands of former revolutionary fighters who take their orders only from their own commanders.
The HRW spokesman appealed to the militia, saying: “They must turn over their detainees to the government. They are undermining the future of Libya as a country ruled by law.
“This is not just about Nick and Gareth. This is about the thousands of Libyans and foreigners who have been picked up by militias who are freelancing with the law.”