He said projectiles landed near the coalition outpost on a hill at the edge of the town.
A US official told the Associated Press a projectile landed near the outpost, but no American troops were reported hurt.
The Kurdish news agency Hawar reported that it was an artillery shell. Both the Observatory and Hawar said US warplanes flew over the base immediately after the shelling.
Turkey’s Defence Ministry said it had taken all measures to ensure no US base was damaged when it responded to harassment fire originating near the outpost.
“There was no firing on the US observation post,” it said of retaliatory fire, which it said was in response to an attack on its military posts south of the town of Suruc, located across the border from the Syrian town of Kobani.
“The firing was ceased as a result of the issue being relayed to us by the US,” the defence ministry said in a statement.
The Chairman of the Joint Chiefs of Staff General Mark Milley told reporters at the Pentagon on Friday that Turkey had been told of US positions in Syria.
“The Turkish military is fully aware – down to explicit grid coordinate detail – of the locations of US forces,” Mr Milley said.
Top Pentagon officials stressed the need for Turkey to avoid doing anything to endanger US forces inside Syria, which numbered about 1,000 before the incursion.
Although US troops had no intention of firing on Turkey, a Nato ally, the Pentagon noted they had the right to defend themselves, if needed.
“Everyone is fully aware that we are the United States military. We retain the right of self-defence,” Mr Milley said.
Donald Trump pulled US troops back from the border this week, saying he wanted American forces out of harm’s way.