Clashes erupted between rival factions on the outskirts of Libya's city of Misrata on Saturday, before being halted by the intervention of another armed force.
The cause of the clashes, near the junction of the main coastal motorway leading to Tripoli, was not immediately clear.
The fighting lasted less than an hour, two residents of the area said, according to Reuters.
A fighter with the force that later moved in to stop the clashes said the two sides had stopped shooting, though there were also reports of fresh mobilisations in the area.
The US ambassador in Tripoli, Richard Norland, gave a warning that the clashes in Misrata "demonstrate the dangerous prospect that the recent violence will escalate”.
“The United States urges all political actors and their supporters among armed groups to stand down in order to avoid escalation and further loss of life," the ambassador said on Twitter.
"Armed efforts either to test or to defend the political status quo risk bringing Libya back to an era its citizens thought had been left behind. Those responsible for such a scenario will be held accountable," he said.
He called on Libya’s political leaders to "immediately embrace an agreed path to elections, which can install a truly legitimate, unified government to serve the interests of all Libyans”.
Western Libya has been tense this year because of a political standoff between the government of Abdul Hamid Dbeibah in Tripoli, and a rival administration under Fathi Bashagha that was appointed by the eastern-based parliament.
Both men are originally from Misrata, a coastal city whose armed factions have played a key role throughout Libya's conflict.
One of the two factions involved in Saturday's clashes is the Joint Operations Force linked to Mr Dbeibah, city residents said. The other is a local group that has not declared an affiliation in the political crisis.
Mr Bashagha has not been able to enter Tripoli because factions linked to Mr Dbeibah have repeatedly stopped him. However, last week amid reports of a realignment of armed groups over the political situation, Mr Bashagha entered Misrata for the first time since his appointment.
Armed forces aligned with each man have since been mobilised around the city, residents have said.
On Friday, troops in Tripoli engaged in the worst clashes there for two years, with 16 people killed including several civilians, authorities said.
The fighting underscored the risk that Libya's political crisis could morph into a new bout of sustained conflict.