HONG KONG — More than a dozen people were killed in the Xinjiang region of far western China this week in a clash between ethnic Uighurs and the police, according to a report by Radio Free Asia that was largely corroborated by local residents, including a police officer.

Radio Free Asia, a news service based in Washington and funded by the United States government that employs Uighur reporters, said that at least 18 and as many as 28 people had been killed in the knife and bomb attack early Monday in Kashgar, a predominantly Uighur city on the ancient Silk Road near China’s border with Kyrgyzstan and Tajikistan. The clash has not been reported in China’s state-controlled news media.

According to the Radio Free Asia report, the violence began when a police officer tried to stop a car that had gone through a checkpoint without stopping. The car backed up, running over the officer’s leg and breaking it. Two unarmed traffic officers who came to help the injured officer were stabbed to death by two people who emerged from the car, Radio Free Asia reported, citing Turghun Memet, a local police officer.

By the time armed police officers reached the scene, more attackers had arrived. Fifteen assailants and three more officers were killed, Radio Free Asia quoted Mr. Memet as saying.