Kurdish officials pushed back against allegations of a Turkish advance, saying their fighters had rejected a land invasion near the city of Tel Abyad overnight.
The Kurdish-led Syrian Democratic Forces, which cooperated with US troops to fight the Islamic State in Syria, said on Thursday that Turkish gunfire had targeted a prison holding some of the jihadist group's fighters in the northeastern city of Qamishli. Thousands of prisoners in the Islamic State and their families are being held in camps and prisons administered by Syrian Kurdish authorities.
In a separate statement on Twitter, SDF spokesman Mustafa Bali said that the Turkish military struck a civilian convoy also near Tel Abyad, about a quarter of a mile from the Turkish border, killing three.
Turkey views Syrian Kurdish fighters as terrorists because of their links with Turkey's Kurdistan Workers Party, or PKK, which has led a decade-long battle in southeastern Turkey for increased independence. It began its long-awaited offensive targeting the SDF in northeastern Syria on Wednesday, with air strikes and fire targeting its outposts along the border.
Mortar fire from Syria landed in at least two Turkish cities, Turkish media told.
The Turkish circle threatens to spread another war-torn Syria, which has been devastated by a year-long conflict.
Mikael Mohammad, a shop owner from Tel Abyad, fled the city with his family Wednesday and slept outdoors in the countryside, he said.
"I just had to leave the clothes I was wearing," he said in a telephone interview. “I immediately got in the car, picked up my family and drove. . . away from the border. "
" Everything I rebuilt over the past few years, maybe I just lost again, "Mohammad said. "The shooting is barbaric and critical."
Syrian Observatory for Human Rights, a British-based war surveillance group, said that 1
6 SDF fighters had been killed since the operation began on Wednesday, including in Ras al-Ayn, which is 75 km east of Tel Abyad.
Nawras, a resident of Ras al-Ayn, described a night of intense shooting. Air strikes resumed in the morning, he said, causing him and his family to flee.
"People still leave Ras al-Ayn as we speak," said Nawras, an electrician. "I am told that the city is still being targeted and that we should not consider going back at this time."
Recent weeks have seen a build-up of Turkish forces on the border, belligerent speeches by Turkish officials and harsh warnings from Turkey's NATO allies and others.
President Trump called the Turkish offensive "a bad idea", but also stood by his decision to withdraw American forces to effectively clear the path for Turkey.
"Turkey is committed to protecting civilians, protecting religious minorities, including Christians, and ensuring that no humanitarian crisis is taking place," he added. "We will hold them to this commitment."
 The offensive has given the Trump administration a dilemma as it has tried to balance Washington's partnership with Turkey and its links to the Syrian Kurdish forces that helped to defeat the Islamic State.
The Erdogan government has been watching nervously for years when Syrian Kurds has built an autonomous enclave along the Turkish border, shaking it against the United States for relying on the Kurds as a military partner and bristling as its enemies accumulated weapons and territory.
For several years, the United States and Turkey have engaged in negotiations which aims to calm Ankaras security problems.  There was also the risk that US troops who are still po sitioned in Syria could get caught in the crossfire.
An American official said that the Trump administration had provided Turkey with a list of places without strikes where US personnel were stationed.
Dadouch and Khattab reported from Beirut.