HASSA, Turkey: Turkish troops and Syrian opposition forces attacked Afrin, a Kurdish enclave in northern Syria Sunday, in their bid to oust from the area a U.S.-allied Kurdish militia, which responded with a hail of rockets on Turkish towns, killing at least one refugee.

The Turkish offensive on Afrin, code-named Operation Olive Branch, started Saturday and threatens to further strain ties between NATO allies Turkey and the U.S.

The United States Sunday urged Turkey to exercise restraint and ensure that the offensive is “limited in scope and duration.” A statement by State Department spokeswoman Heather Nauert also asked Turkey to be “scrupulous to avoid civilian casualties,” adding that all parties involved in Syria should focus on defeating Daesh (ISIS).

The Syrian government, Iran and Egypt condemned the attack, which activists said has killed at least 18 civilians in the Kurdish-held enclave, Afrin, in the first 24 hours.

Turkish officials say 11 rockets launched from Syria have landed in Turkish towns along the border, killing at least one Syrian refugee and injuring 47.

France called for an emergency U.N. Security Council meeting to discuss the developments there and urged Turkish authorities “to act with restraint in a context where the humanitarian situation is deteriorating in several regions of Syria.”

Turkish officials said the troops entered Afrin a day after dozens of Turkish jets and artillery units at the border pounded Syrian Kurdish targets. A spokesman for the Kurdish fighters said the attack was repelled.

Turkey gave Washington advance warning before launching an operation against U.S.-allied Syrian Kurdish forces, and Ankara has “legitimate” security concerns, Defense Secretary Jim Mattis said.

“Turkey was candid. They warned us before they launched the aircraft they were going to do it in consultation with us, and we are working now on the way ahead through the Ministry of Foreign Affairs,” Mattis told reporters aboard his aircraft at the start of a trip to Asia. U.S. officials have said that the administration had appealed to Turkey not to go ahead.

The operation, for which Turkey has also rallied nearly 10,000 Syrian opposition fighters, could spill into a wider Turkish-Kurdish confrontation inside Turkey. There are an estimated 8,000 to 10,000 Kurdish fighters in the Afrin district, the Turkish prime minister said.

The operation also includes airstrikes on the district, threatening to create another humanitarian disaster in the region.

The Afrin district houses no fewer than 800,000 civilians, including displaced people from earlier years of the Syrian war. Russia pulled back troops that had been deployed near Afrin after it was briefed on the operation by Turkey.

Badran Ciya Kurd, an adviser to the Kurdish administration in northern Syria who meets regularly with Russian and U.S. officials, told the Associated Press Sunday that Russian officials suggested that handing over the enclave, encircled by Syrian government and its rival Turkey and Syrian fighters it backs, would avert the Turkish offensive.

It was not immediately possible to reach Russian officials.

Prime Minister Binali Yildirim told a group of journalists that Turkey aims to create a 30-kilometer-deep “secure zone” in Afrin.

The state-run Anadolu Agency said Sunday that the Turkish-backed fighters had penetrated 5 kilometers into Afrin as part of the offensive. At least one person, a Syrian refugee in Turkey, was killed when Reyhanli, a Turkish border town, came under a hail of rockets.

It was the second Turkish town to come under attack.

Earlier, the rockets fired from Syria targeted the border town of Kilis, but there were no casualties.

SDF spokesman Mustafa Bali denied that Turkish troops had entered Afrin, saying Kurdish forces have been repelling attacks since Saturday. Bali said the SDF sent reinforcements to Afrin. The YPG said meanwhile that it had destroyed two Turkish tanks.

The Britain-based Syrian Observatory for Human Rights said that Kurdish militia and Turkish forces clashed on the northern and western edges of Afrin. It said the Turkey-backed forces entered Shinkal and Adah Manli to the west and that Turkey-backed forces captured three YPG fighters. The Observatory said airstrikes killed eight in Afrin’s southeast, bringing the total of civilians killed since the attack began to 18.