Turkish forces are staging an offensive targeting a US-backed Kurdish group in northern Syria, local media have reported.
Warplanes bombed the city of Afrin, which is controlled by the Kurdish People's Protection Units (YPG), early on Sunday morning.
Ground troops have now crossed the border into the Kurdish-run enclave, according to Turkish media reports.
The Turkish attack - codenamed Operation Olive Branch - came as President Recep Tayyip Erdogan promised to expand Turkey's military border operations in its fight against Islamic State.
A military statement said the operation was intended to "neutralize" Syrian Kurdish fighters in Afrin and to save the local population from their "pressure and oppression".
But the operation has received condemnation from some quarters - Egypt described that attack as a breach of Syria's sovereignty and said the assault threatens political negotiations.
Iran's official IRNA news agency also reported that the country's foreign ministry has condemned the assault and expressed its concerns over this operation.
IRNA quotes the ministry spokesman Bahram Ghasemi as calling on Turkey to end the offensive immediately.
"The continuing crisis in Afrin may boost terrorist groups again in the northern parts of Syria," he was quoted as saying.
The YPG are US allies in the fight against IS, but they are treated as terrorists by Turkey.
The Kurds have called for an independent state which they want to be carved out of territory across both Syria and Turkey.
Leaders in Ankara have been infuriated by an announcement from the US military a week ago that it was going to create a 30,000-strong border force with the Kurdish fighters to secure northern Syria.
With American support, including close to 2,000 embedded forces, the coalition now controls close to a quarter of Syrian territory, concentrated mostly in the north-east.
Days later, US secretary of state Rex Tillerson announced that the US would maintain a military presence with the Kurds for the foreseeable future.