Troops loyal to Libyan military commander Khalifa Haftar said Tuesday they shot down a warplane and captured its foreign "mercenary" pilot near the capital Tripoli.

The Mirage F1 was downed in Al-Hira region, 70 kilometres (43 miles) south of the capital, Haftar's self-styled Libyan National Army said on its Facebook page.

There was no immediate comment on the report from the Government of National Accord, whose forces have been locked in battle with the LNA since Haftar's fighters launched an offensive to take Tripoli on April 4.

The LNA posted pictures of a man with an apparent head wound and blood-splattered uniform whom it identified as a "Portuguese mercenary".

In a video circulating on social media, the man is asked in English by an LNA fighter in the back of a pickup truck whether he was a soldier. "No, I am a civilian," he says.

In another video published by a pro-Haftar website, the pilot says he is a 29-year-old "from Portugal".

"I was requested to destroy roads and bridges" as part of a "civilian contract", he said in English in response to a question about what he was doing.

Asked about the incident by AFP, a spokesman for the Portuguese defence ministry said the country's "air force has no missing pilots, is not currently on a mission in Libya and does not have a Mirage F1 in its fleet".

The almost five-week-old battle on the southern outskirts of Tripoli has killed at least 432 people, wounded 2,069 and displaced some 55,000 others, according to the UN.

After initial advances for the LNA, its rivals have launched a counter-offensive, leading to a stalemate on the ground.

The two sides often accuse each of resorting to the use of foreign mercenaries and outside military assistance.

Libya has been mired in chaos since the ouster and killing of Moamer Kadhafi in 2011, with Haftar supporting a rival administration in the east of the country.