Ceylanpinar, Oct 12 (AP) Turkey's military said it captured a key Syrian border town under heavy bombardment Saturday in its most significant gain since an offensive against Kurdish fighters began four days ago, with no sign of relent despite mounting international criticism.

Turkish troops entered central Ras al-Ayn, according to Turkey's Defence Ministry and a war monitor group.

The ministry tweeted: "Ras al-Ayn's residential center has been taken under control through the successful operations in the east of Euphrates" River. It marked the biggest gain made by Turkey since the invasion began Wednesday.

The continued push by Turkey into Syria comes days after President Donald Trump cleared the way for Turkey's air and ground offensive, pulling back US forces and saying he wanted to stop getting involved with "endless wars."

Trump's decision drew swift bipartisan criticism that he was endangering regional stability and risking the lives of Syrian Kurdish allies.

The Kurdish-led Syrian Democratic Forces was the main US ally in the fight and lost 11,000 fighters in the nearly five-year battle against IS.

Turkish troops and allied Syrian opposition fighters have made gains recently capturing several northern villages.

The invasion also has forced nearly 100,000 people to flee their homes amid concerns that IS might take advantage of the chaos.

The Syrian Democratic Forces or SDF called on the United States to carry out its "moral responsibilities" and close northern Syrian airspace to Turkish warplanes.

"We don't want them to send their soldiers to the front lines and put their lives in danger," the statement said.

"What we want is for them" to close the airspace for Turkish warplanes.

During a meeting Saturday in Cairo, the 22-member Arab League condemned what it described as "Turkey's aggression against Syria" and warned that Ankara will be responsible for the spread of terrorism following its invasion.

It called on the UN Security Council to force Turkey to stop the offensive.

Turkish President Recep Tayyip Erdogan said Friday that Turkey won't stop until the Syrian Kurdish forces withdraw below a 32 kilometer (20 mile) deep line from the border.

During the capture of Ras al-Ayn's residential centre, an Associated Press journalist across the border heard sporadic clashes as Turkish howitzers struck the town and Turkish jets screeched overhead.

Syrian Kurdish forces appeared to be holding out in some areas of the town.

The SDF released two videos said to be from inside Ras al-Ayn, showing fighters saying that it was Saturday and they were still there.

The fighting was ongoing as the Kurdish fighters sought to reverse the Turkish advance into the city, the Syrian Observatory for Human Rights said.

Ras al-Ayn is one of the biggest towns along the border and is in the middle of the area where Turkey plans to set up its safe zone.

The ethnically and religiously mixed town with a population of Arabs, Kurds, Armenians and Syriac Christians had been under the control of Kurdish fighters since 2013.

IS members tried to enter Ras al-Ayn following their rise in Syria and Iraq in 2014 but failed.

Earlier Saturday, Turkish troops moved to seize control of key highways in northeastern Syria, the Turkish military and the Syrian Observatory said.

Turkey's state-run Anadolu news agency said that Turkey-backed Syrian opposition forces had taken control of the M-4 highway that connects the towns of Manbij and Qamishli.

Kurdish news agencies including Hawar and Rudaw said that Hevreen Khalaf, secretary general of the Future Syria Party, was killed Saturday as she was driving on the M-4 highway.

Rudaw's correspondent blamed Turkish forces for targeting Khalaf's car, and Hawar blamed "Turkey's mercenaries."

The observatory said six people, including Khalaf, were killed by Turkey-backed opposition fighters on the road that they briefly cut before withdrawing.

Since Wednesday, Turkish troops and Syrian opposition fighters backed by Ankara have been advancing under the cover of airstrikes and artillery shelling.

Turkey has said it aims to push back the Syrian Kurdish People's Protection Units or YPG, which it considers terrorists for links to a decades-long Kurdish insurgency.

The YPG is a main component of the SDF.

The UN estimated the number of displaced at 100,000 since Wednesday, saying that markets, schools and clinics also were closed.

Aid agencies have warned of a humanitarian crisis with nearly a half-million people at risk in northeastern Syria.

A civilian wounded in a mortar strike from Syria on Friday in the Turkish border town of Suruc died, Anadolu news agency reported Saturday, bringing the civilian death toll to 18 in Turkey.

Turkey's interior minister said hundreds of mortars, fired from Syria, have landed in Turkish border towns.

The observatory that keeps track of Syria's civil war said 74 Kurdish-led SDF fighters have been killed since Wednesday as well as 49 Syrian opposition fighters backed by Turkey.

That's in addition to 38 civilians on the Syrian side. It added that Turkish troops now control 23 villages in northeastern Syria.

Turkey's defence ministry said it "neutralised" 459 Syrian Kurdish fighters. The number could not be independently verified.

Four Turkish soldiers have been killed since the beginning of the offensive.

Kurdish fighters are holding about 10,000 IS fighters, including some 2,000 foreigners. (AP)

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