Turkish forces claimed to have taken control of a key Syrian border town Saturday as their assault against Kurdish fighters in the region continued.The Turkish Defence Ministry said that it had seized Ras al-Ayn as part of the operation, code-named ‘Peace Spring,’ which began Wednesday.NBC News has been unable to independently verify the claim.The Kurdish-led Syrian Democratic Forces (SDF) denied the town had been captured and appeared to be holding out in some areas, the Associated Press reported.Turkey’s push deeper into Syria comes amid widespread international criticism, fears of a resurgence of the Islamic State group and humanitarian concern for those displaced by the offensive.The capture of the town would represent Ankara’s most significant gain since President Donald Trump said he had decided to pull U.S. troops back from the area to clear the way for Turkish forces.The move was met with criticism, which was only likely to intensify after U.S. troops stationed in the region came under artillery fire from Turkish forces late Friday.An explosion occurred within a few hundred yards outside a security zone known by the Turks to have U.S. forces present, Navy Capt. Brook DeWalt, director of Defense Press Operations, told NBC News.No American troops were injured in the incident, near Kobani in northeastern Syria.The U.S. is now warning Turkey to avoid any actions that could result in “immediate defensive action,” DeWalt said.RelatedTurkish troops are fighting the Syrian Democratic Forces, which are led by the Kurdish People’s Protection Units (YPG). Turkey views the YPG as an extension of the separatist Kurdistan Workers’ Party (PKK), which is considered a terrorist group by the United States.But the Kurdish-led Syrian Democratic Forces (SDF) have been crucial U.S. allies in the war on the Islamic State militant group.The SDF has held more than 10,000 ISIS members in detention centers and prison camps, but they said they are being forced to abandon some of those positions to fight the Turkish invasion.They reported that a facility in the border town of Qamishli was struck by Turkish artillery on Friday and some prisoners had attempted to escape.But Turkish-backed Syrian opposition forces continued to push ahead, the Anadolu news agency reported early Saturday. They reached a strategic highway that connects the Syrian towns of Manbij and Qamishli — the de facto capital of Syrian Kurdistan.At least 100,000 people have been displaced within the first three days of the attack, according to the United Nations’ Humanitarian Affairs Office.The number of casualties since the violence began has varied between sources and NBC News has been unable to independently verify any claims.Turkey’s defense ministry stated early Saturday that 415 “terrorists” have been killed since launching its military operation.The Syrian Observatory for Human Rights said Saturday that at least 21 civilians have been killed, including six children.The U.S. set down red lines for the offensive Thursday that would trigger economic sanctions, including ethnic cleansing and indiscriminate fire directed at civilian populations.“If Turkey acts in a way that is disproportionate, inhumane, or otherwise goes beyond the lines the President has, in his own mind, the United States is willing to impose significant cost,” said a senior State Department official, who spoke on the condition of anonymity because they were not authorized to speak publicly on the matter.Linda GivetashLinda Givetash is a reporter based in London. She previously worked for The Canadian Press in Vancouver and Nation Media in Uganda. Aziz Akyavas , Mac William Bishop, Mosheh Gains, Nancy Ing and Associated Press contributed.