Pompeo placed a phone call to Prime Minister Fayez al-Sarraj to "reiterate US opposition to the continued level of weapons and munitions being brought into the country," the State Department said.
Pompeo and Sarraj "emphasized the importance of an immediate halt to the fighting and return to political dialogue," a statement said.
Sarraj's UN-recognized Government of National Accord has scored a series of successes in recent weeks in pushing back against warlord Khalifa Haftar, including seizing a strategic airfield near Tripoli on Monday.
Haftar -- who is backed by US allies Egypt, the United Arab Emirates and Saudi Arabia -- launched an offensive in April 2019 to seize Tripoli that has killed hundreds.
A recent UN report on Libya's widely violated arms embargo also showed support for Haftar by mercenaries from a Russian security group seen as close to President Vladimir Putin.
Pompeo's statement did not name any country for sending in weapons, but the Government of National Accord's key military supplier is Turkey, which signed a pact with Tripoli in November.
A report last month by the International Crisis Group said that Turkey has sent into Libya at least 100 military officers, shiploads of weapons and aerial defenses as well as at least 2,000 pro-Turkish fighters from Syria.
The United States officially backs the UN-recognized government and opposes Haftar's offensive.
But its position has caused confusion as President Donald Trump, who has close ties with Haftar's Arab allies, in April 2019 spoke with the strongman by telephone and praised him.
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