AKCAKALE, Turkey (AP) — Turkish forces faced fierce resistance from U.S.-allied Syrian Kurdish fighters on the third day of Ankara’s offensive in northern Syria, as casualties mounted, international criticism of the campaign intensified and estimates put the number of those who fled the violence at 100,000.

In a complicating twist, Washington said its troops also came under fire from NATO ally Turkey.

An explosion was reported later Friday in northern Syria near an outpost where U.S. troops are located, but none of the Americans were hurt, according to a U.S. official and a Syria war monitor. It was unclear whether it was from artillery or an airstrike, and it was the first time a coalition base was in the line of fire since Turkey’s offensive began.

Turkey said the U.S. was not targeted and its forces were returning fire after being targeted by Kurdish fighters about half a mile from the U.S. outpost. The Turkish Defense Ministry said it ended the strike after communicating with the U.S.

Navy Capt. Brook DeWalt, a Pentagon spokesman, says the artillery explosion came within a few hundred meters of the area where U.S. troops were.

The artillery strike so close to American forces showed the unpredictable nature of the conflict days after U.S. President Donald Trump said he was getting U.S. troops out of harm’s way.

Earlier, Turkey said it captured more Kurdish-held villages in the border region, while a hospital in a Syrian town was abandoned and a camp of 4,000 displaced residents about 7 miles from the frontier was evacuated after artillery shells landed nearby.