Faraj al-Swehli, commander of a Misrata brigade, said the men had entered Libya illegally and were carrying “incriminating evidence”.
He said the activities of reporter Nicholas Davies, 37, and cameraman Gareth Montgomery-Johnson, 36, from Carmarthen, were being investigated.
The men had been working for Iran’s English-language TV station Press TV.
Rights groups have called for their release.
The BBC’s Gabriel Gatehouse in Tripoli says that at a hastily convened press conference in the capital, reporters were shown video footage of what was purported to be the two journalists test-firing weapons.
Members of the militia also produced a field dressing that they said they had found in the journalists’ possession.
It was of a type, they said, used by the Israeli military.
Mr Swehli told reporters that he believed the two men were spies and that he and his men were conducting their own investigation into the matter.
“After we have finished the investigation we are going to transfer them to the state authorities to pursue the legal process against them,” he said, according to Reuters.
Mr Swehli said the two men were being well treated.
Mr Davies, who works under the name Nick Jones, and Mr Montgomery-Johnson were detained on 21 February, apparently while filming late at night on the streets of Tripoli.
The initial accusation against them was that their immigration documents were not in order.
The Libyan Prime Minister’s office and the interior ministry told the BBC that they knew nothing of the allegations of spying.
All efforts by the interim government and the British embassy to persuade the militia to release the journalists into official custody have so far met with refusal.