BEIRUT — Intense clashes erupted Monday as Turkish troops and their allies advanced on a Kurdish enclave in Syria, the third day of the Ankara offensive aimed at ousting the US-backed Kurdish militia from the area, the militia and a war monitoring group said.

The Turkish offensive on Afrin, codenamed Operation Olive Branch, started on Saturday, heightening tensions in the already complicated Syrian conflict and threatening to further strain ties between NATO allies Turkey and the United States. Turkey says it aims to to create a 30-kilometer (20-mile) deep "secure zone" in Afrin.

The UN Security Council was to convene later Monday to discuss the situation in Syria.

A NATO statement Monday said it has been in touch with Turkey over the developing offensive. NATO said Turkey has suffered from terrorism and has the right to self-defense but urged Ankara to do so in a "proportionate and measured way."

NATO also said it has no presence in Syria but that as members of the coalition against Islamic State militants, "our focus is on the defeat" of the extremist group.

The US-backed Kurdish militia said it has repelled Turkish troops and their Syrian allies from Shinkal and Adah Manli, two villages they seized a day earlier in Afrin, the northwestern Syrian district that straddles the Turkish border.

The militia said the Turkey-backed forces have opened a new front, pushing their way into two other villages in the district's north. The militia said they are fighting to push back the advancing troops in Balia and Qarna.

The Syrian Observatory for Human Rights said the Syrian Kurdish militia waged a ferocious counteroffensive late Sunday, repelling the Turkish troops and allied Syrian fighters from the two villages they briefly captured. The Observatory said Turkey-backed troops were attempting once again to enter Afrin.

Access to Afrin is restricted and it is difficult to independently verify the reported developments.

The Kurdish militia, known as People's Defense Units or YPG, said it was clashing Monday with the Turkish troops northwest of Afrin.

Turkey considers the YPG a terror organization because of its affiliation to its own Kurdish insurgency. Turkish President Recep Tayyip Erdogan has promised to expand the operation. The militia formed the backbone of the Syrian Democratic Forces, US' ally in the war against the Islamic State militants in Syria. — AP