Air raids by the self-styled Libyan National Army against the capital last night killed four people and wounded 20 others, Libya's internationally recognised unity government has said.

But Amin al-Hachemi, a spokesman for the Government of National Accord's health ministry, has warned that "the death toll could increase in the coming hours".

A pro-GNA military source said the victims were civilians.

"Several sites were targeted by air strikes late last night, causing victims among civilians," the source told AFP.

"Most of the strikes hit areas in the district of Abou Slim... (but) none hit military targets."

Strongman Khalifa Haftar's Libyan National Army  launched an offensive against Tripoli, the seat of the Government of National Accord, on 4 April .

However, after initial gains, Haftar's forces have encountered stiff resistance on the southern outskirts and his troops have been pushed back in some areas.

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At least 278 people have been killed and more than 1,300 wounded in the clashes, according to a toll released on Wednesday by the World Health Organization.

The GNA accuses Khalifa Haftar of using foreign planes to carry out air strikes, without naming a country of origin.

Migrants in Libya have found themselves trapped in the fighting that has raged for three weeks

"This criminal conceals his failures and those of his soldiers at the gates of Tripoli by resorting to foreign aviation to hit unarmed civilians in the city," spokesman Mohanad Younes said on the GNA's official Facebook page.

Strongman Haftar's offensive has sharpened fault lines in policy towards Libya among world powers.

On 18 April, Russia and the United States opposed a British bid backed by France and Germany at the UN Security Council to demand a ceasefire in the North African country.

The White House revealed the next day that Donald Trump had reached out personally to Khalifa Haftar in a phone call, during which the US president "recognised Field Marshal Haftar's significant role in fighting terrorism and securing Libya's oil resources".

The country has been mired in chaos since the NATO-backed uprising that deposed and killed dictator Moamer Kadhafi in 2011.