CAIRO (AP) — Deadly clashes between rival militias in Libya’s capital killed at least 27 people and left residents trapped in their homes on Tuesday, unable to escape the violence, medical authorities said.

The fighting appears to be the most intense to shake Tripoli this year. There were at least four people dead but it was not immediately clear if they were militiamen or civilians, an official said.

The clashes erupted late on Monday between militiamen from the 444 brigade and the Special Deterrence Force, according to local media reports. Tensions flared after Mahmoud Hamza, a senior commander of the 444 brigade, was allegedly detained by the rival group at an airport in Tripoli earlier in the day, the reports said.

Over 100 people were injured in the fighting, Libya’s Emergency Medicine and Support Center, a medical body that is deployed during humanitarian disasters and wars, said early Wednesday.

It is unclear how many of the dead were militiamen or civilians. The Red Crescent did not immediately respond to a request for comment.

Throughout the fighting Tuesday, the Health Ministry urged the warring sides to allow ambulance and emergency teams to enter the affected areas, primarily in the south of the city, and for blood to be sent to nearby hospitals.

OPSGroup, an organization for the aviation industry, said late Monday that a large number of aircraft departed from Tripoli due to the clashes. Inbound flights were being diverted to the nearby city of Misrata, it said.

The escalation follows months of relative peace after nearly a decade of civil war in Libya, where two rival sets of authorities are locked in a political stalemate. Longstanding divisions have sparked several incidents of violence in Tripoli in recent years, although most have been over in a matter of hours.

In a statement Tuesday, the U.N. mission in Libya said it was following with concern “the security incidents and developments” and called for an immediate end to the ongoing clashes.

Both of Libya’s rival administrations also condemned the fighting in separate statements Tuesday. The House of Representatives, which is based in the eastern city of Benghazi, blamed its rival, the Tripoli-based government, for the violence.

The U.S. and British embassies in Libya issued statements expressing concerns over the violence. The United States called for an “immediate de-escalation in order to sustain recent Libyan gains toward stability and elections,” the American Embassy said.

The oil-rich country has been divided since 2014 between rival administrations in the east and the west, each supported by an array of well-armed militias and different foreign governments. The North African nation has been in a state of upheaval since a 2011 NATO-backed uprising toppled and later killed longtime dictator Moammar Gadhafi.

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Associated Press writer Jon Gambrell in Dubai, United Arab Emirates, contributed to this report.