The White House has revealed that US President Donald Trump reached out personally to Libyan warlord Khalifa Haftar, as a push at the United Nations to broker a ceasefire hit trouble.

Observers see Mr Trump's words of praise for the strongman, at the expense of internationally-recognised leader Fayez al-Sarraj, as evidence of US support that explains Mr Haftar's determination to pursue his offensive to seize Tripoli.

The two men spoke on Monday "to discuss ongoing counter-terrorism efforts and the need to achieve peace and stability in Libya," according to the White House.

A statement said that Mr Trump "recognised Field Marshal Haftar's significant role in fighting terrorism and securing Libya's oil resources," adding that "the two discussed a shared vision for Libya's transition to a stable, democratic political system."

The White House did not say why it delayed giving news of the phone call.

Mr Haftar, seen by his allies Egypt and the United Arab Emirates as a bulwark against Islamists, has declared he wants to seize the capital, now controlled by a UN-recognised government and an array of militias.

The military commander backs a rival administration based in eastern Libya that is refusing to recognise the Tripoli government's authority.

On Thursday, Russia and the United States opposed a British-bid backed by France and Germany at the UN Security Council to demand a ceasefire in Libya.

Russia insisted on having no criticism of Mr Haftar in the proposed resolution, while the US said it wanted more time to consider the situation.

Diplomats say the signaling from Washington goes a long way towards explaining Mr Haftar's aggressive strategy in the face of strong condemnation by the European powers and the UN. 

"Haftar believes he has to fight until the end," said one diplomat at the UN speaking on the condition of anonymity.