HASSA, Turkey — Turkish troops and Syrian opposition forces attacked a Kurdish enclave in northern Syria on 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, codenamed Operation Olive Branch, started Saturday and has heightened tensions in the already complicated Syrian conflict, threatening to further strain ties between NATO allies Turkey and the United States.

On Sunday, the United States 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 the Islamic State group.

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 considers the Syrian Kurdish militia, known as the People’s Protection Units, or YPG, a terror organization and a security threat because of its affiliation with Kurdish rebels fighting in southeastern Turkey.

The group controls Afrin, in Syria’s northwestern Aleppo province, as well as nearly 25 percent of Syrian territory, to the east along Turkey’s border. The YPG also forms the backbone of the Syrian Democratic Forces, the main U.S. ally against the Islamic State group in Syria.

U.S. support for the Kurdish militia has been a cause of perpetual conflict between Ankara and Washington, which has backed the Kurdish militia.

U.S. officials have said that the administration had appealed to Turkey not to go ahead with the offensive. A Turkish operation there could have an impact on U.S. operations further east in Syria, the officials said.

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 is 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 less 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.

Kurdish officials said Russian military officials have proposed handing over Afrin to the Syrian government to avert a Turkey military offensive. They said they refused the proposal.

Badran Ciya Kurd, an adviser to the Kurdish administration in northern Syria who meets regularly with Russian and US 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 (20-mile) deep “secure zone” in Afrin.

On Sunday, the state-run Anadolu Agency said the Turkish-backed fighters had penetrated 5 kilometers (3 miles) 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 Sunday.

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.

In a statement, the Syrian opposition fighters battling alongside the Turkish troops said the combined force seized Shinkal, a village on the northwestern edge of Afrin district. A Syrian rebel commander said the clashes with the YPG fighters were intense, but that the Turkey-backed forces would fight to “eliminate terrorism” from the area.

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. It added that the 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.

Sepan Jan, a journalist in Afrin, said stores and services were operating normally. Residents in border villages were taking precautions against the bombings, he said. The only road out of Afrin, leading to government-controlled Aleppo, has been closed for security reasons, he added.

Turkish President Recep Tayyip Erdogan has vowed to expand the offensive to Manbij, a town to the east that Kurdish forces seized from IS in a costly battle with the aid of the U.S.-led coalition. The town has since emerged as a model for U.S.-backed Kurdish rule of largely Arab areas.

A Turkish advance on Manbij would further strains relations with Washington, which has troops operating in the Manbij area.

Turkish troops first crossed into Syria after the Kurds captured Manbij in 2016, in part to prevent them from expanding westward and linking territory to Afrin. At least 70 Turkish soldiers were killed, most in battles with IS militants, which have since been driven from nearly all the territory it once held in Syria.

Syria’s government had vowed to shoot down any Turkish fighter jets over Afrin, calling it an “aggressive act.” On Sunday, President Bashar Assad condemned the “brutal aggression” on Afrin but didn’t repeat the threat. He said Turkey has always supported “terrorists” in Syria.

Iran, a close ally of Assad, also condemned the Turkish assault and called on Turkey to end it.

“The continuing crisis in Afrin may boost terrorist groups again in the northern parts of Syria,” Iranian Foreign Ministry spokesman Bahram Ghasemi said according to Iran’s official IRNA news agency.

Egypt, which maintains security coordination with Syria and is at odds with Turkey, said the military offensive threatened political negotiations.

Turkey has prepared around 10,000 Syrian fighters to storm Afrin, according Rami Abdurrahman, director of the Syrian Observatory for Human Rights. Some were stationed in Azaz, on the eastern edge of Afrin and others in Atmeh to the south.

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El Deeb reported from Beirut. Associated Press writer Suzan Fraser in Ankara, Turkey contributed to this report.

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