The White House says President Trump has spoken with Libyan eastern commander General Khalifa Haftar, whose troops are attacking the capital, Tripoli.
During the call on Monday, Trump recognized Gen Haftar's efforts to combat terrorism and safeguard Libya's oil for Libya's future.
Tripoli is the seat of the UN-backed and internationally recognized government in Libya.
In Mr. Trump's request, he states that he supports Gen Haftar unlike some of his allies.
Since then, more than 200 people have been killed The fighting started three weeks ago.
On Thursday, UN-backed Prime Minister Fayez al-Serraj condemned the "silence" of his international allies during the Gen Haftar forces' attack.
Libya was torn apart by violence and political instability for a long time ruler Muammar Gaddafi was deposed and killed in 201
- Why is Libya so lawless?
- Who is the military man Khalifa Haftar?
The last crisis began three weeks ago, when Gen Haftar's eastern forces collapsed in the capital called by Mr Serraj as an attempted coup d'état. [Gen19659007] Gen Haftar's troops are moving from different directions on the outskirts, claiming they have conquered Tripoli's international airport. 19659017] Does that mean that Trump is behind Haftar?
During Mr. Trump's meeting with Gen Haftar, "the two discussed" a shared vision for Libya's transition to a stable, democratic political system, "says the White House.
The BBC Center Alan Johnston, East Regional Editor, says the call appears to signal that Washington is swinging its weight behind Gen Haftar, and may consider it capable of restoring the country's unity and order.
But Gen Haftar's opponents say he would rule the country A very autocratic style, adds our correspondent.
The US and Russia have also refused to support a UN-sponsored United Nations resolution calling for a ceasefire in Libya.
Russia disagrees with blaming Gen Haftar for the violence, while the US had not given a reason, Reuters reported.
Gen Haftar had support from the United Arab Emirates and Egypt and visited Saudi Arabia shortly before his attack on Tripoli.
The UN-backed government in Tripoli has also accused France of supporting Gen Haftar. France has denied this.
Reuters quoted Jalel Harchaoui of the Clingendael Institute in The Hague as saying that Trump's call was tantamount to supporting Gen Haftar's campaign and making military intervention by an outside state like Egypt more likely.
Who supports the Government of Tripoli? 19659018] The former colonial power Italy supports the internationally recognized government.
British Foreign Minister Jeremy Hunt has also said there is "no reason" for Gen. Haftra's move to Tripoli.
On Friday, Hunt spoke with US Secretary of State Mike Pompeo, and the two agreed to "continue diplomatic efforts to freeze locally and return to the political process," said the State Department.
Who is General Haftar?
A former Army officer, he helped Colonel Gaddafi take power in 1969 before leaving with him and exiled to the United States.
He returned in 2011 after the uprising against Gaddafi and became a rebel commander.
After Gaddafi's case, he was appointed head of the Libyan National Army (LNA) under a former UN-backed government.