Turkish troops and tanks entered Syria on Sunday to push an offensive against Kurdish militia as rockets hit border towns in apparent retaliation and the United States urged Ankara to show restraint.
Turkey on Saturday launched operation "Olive Branch" seeking to oust from the Afrin region of northern Syria the Peoples' Protection Units (YPG) which Ankara considers a terror group.
But the campaign risks further increasing tensions with Turkey's NATO ally Washington, which has supported the YPG in the fight against Islamic State (IS) jihadists and warned Ankara about distracting the focus from that fight.
In its first reaction to the offensive, the US State Department urged Turkey Sunday "to exercise restraint and ensure that its military operations remain limited in scope and duration and scrupulous to avoid civilian casualties".
Defense Secretary Jim Mattis said Ankara had given Washington advanced warning of their operation, adding Turkey's security concerns were "legitimate".
Turkish Prime Minister Binali Yildirim said troops crossed into YPG-controlled region in Syria at 0805 GMT, the Dogan news agency reported.
Thirty-two Turkish planes destroyed a total of 45 targets including ammunition dumps and refuges used by the YPG on the second day of the operation, the Turkish army said.
Turkish troops were advancing alongside forces from the Ankara-backed rebel Free Syrian Army (FSA) and were already five kilometers inside Syria, state media said.
A photographer saw more Turkish tanks lined up at the border waiting to cross into Syrian territory.
Foreign Minister Mevlut Cavusoglu said in televised comments several villages had already been taken in the advance.
But a YPG spokesman claimed Turkish forces seeking to enter Afrin had been "blocked" and that it had hit two Turkish tanks.
The Britain-based Syrian Observatory for Human Rights said a total of 18 civilians had been killed so far in the two-day operation, including eight members of the same family sheltering in a building that was flattened by an airstrike.
The YPG confirmed the deaths, sending out pictures of Kurdish Red Crescent rescuers retrieving bloodied bodies from a collapsed concrete structure.
But Ankara denied any civilian casualties, with Cavusoglu accusing the YPG of sending out "nonsense propaganda and baseless lies".
Top: Turkish armoured personnel carriers (APCs) and infantry fighting vehicles (IFVs) are seen driving near the village of Yazi Bagh, about six kilometres from the Bab al-Salamah border crossing between Syria and Turkey in the north of Aleppo province, on January 21, 2018, with the flags of the Syrian opposition seen flying in the background. (Photo: VCG)