CAIRO — Libyan rebels have seized control of a key coastal city, the country’s beleaguered government conceded on Tuesday, amid new criticism that the growing role of foreign powers in the chaotic conflict has fueled a sudden escalation in fighting.

Forces loyal to the militia leader Khalifa Hifter swept into the city, Surt, on Monday, days after Turkey announced that it was deploying troops to prop up the weak United Nations-backed government.

The fall of Surt is a significant blow to the government, which has been under siege from Mr. Hifter’s forces since April.

In 2016, American warplanes carried out hundreds of airstrikes against Islamic State fighters in Surt, in support of a government force that eventually expelled the group from the city. Now that the government has relinquished that gain, this time to Mr. Hifter’s forces, its authority has shrunk to a 250-mile strip of coastline stretching from either side of the capital, Tripoli.