A man has reportedly been killed in a rare confrontation in north-eastern Syria between US-led coalition forces and pro-government militia fighters. Syria's state news agency, Sana, said a civilian died when US troops fired at a crowd stopping them pass through a checkpoint east of Qamishli city. A monitoring group said it was not clear if the dead man was armed or not. The coalition said a patrol came under fire from "unknown individuals" and returned fire in self-defence. About 500 US troops are in north-eastern Syria to assist the Kurdish-led Syrian Democratic Forces (SDF) alliance in the fight against the Islamic State (IS) group. Their mission has become more complicated since October, when the US announced a troop drawdown and Turkey launched a military offensive that forced Kurdish SDF fighters to pull back from Syrian border areas they controlled. A ceasefire agreement that halted the offensive led to Syrian pro-government, Russian and Turkish forces moving into territory along the border. US troops meanwhile moved to Syria's far north-eastern tip, where Qamishli is located, and south to Deir al-Zour province to safeguard SDF-controlled oilfields. Coalition spokesman Col Myles Caggins said its forces were conducting a patrol near Qamishli on Wednesday when they encountered a checkpoint manned by Syrian pro-government forces. "After coalition troops issued a series of warnings and de-escalation attempts, the patrol came under small arms fire from unknown individuals. In self-defence, coalition troops returned fire," he added, without mentioning any casualties. "The situation was de-escalated and is under investigation. The coalition patrol returned to base." A Sana reporter said the deadly clash occurred after hundreds of people from the villages of Khirbet Ammo and Hamou gathered to stop four coalition vehicles passing through a Syrian army checkpoint. "The American occupation soldiers fired live bullets and smoke bombs on the locals as a civilian from the village of Khirbet Ammo was martyred while another civilian from Hamou village was injured," the reporter added. The crowd retaliated by attacking and damaging the four coalition vehicles before reinforcements arrived and the troops withdrew, Sana said. Later, it added, there was a coalition air strike on Khirbet Ammo and clashes involving light weapons erupted between the locals and US troops. The Syrian Observatory for Human Rights, a UK-based monitoring group, reported that residents and pro-government militiamen in Khirbet Ammo tried to block a coalition patrol and that militiamen fired into the air. In response, coalition troops fired smoke bombs and then live rounds, killing one of person, it said, adding that it was not yet clear if he was a civilian or a militiaman. The Syrian Observatory and activists from the pro-Kurdish Rojava Information Center also reported that US aircraft carried out a strike in the Qamishli area.