Turkish forces and Syrian rebels fought government troops in northwest Syria on Thursday and Russian warplanes struck back in an sharp escalation of an already intense battle over the last rebel bastions, Russian and Turkish officials said.
The Turkish defence ministry said more than 50 Syrian soldiers had been killed near Idlib city in retaliation for air strikes that had killed two Turkish soldiers and wounded five.
Earlier, talks between Moscow and Ankara, who back different sides in Syria's nine-year-old war, had failed to reach a compromise to ease the fraught situation and head off a direct confrontation between them in Aleppo and Idlib provinces.
In Geneva, the United Nations refugee chief called for a halt to the fighting to allow hundreds of thousands of trapped and destitute civilians to move to places of safety.
Thursday's action took place a day after Turkish President Tayyip Erdogan threatened to unleash a military operation against the Syrian government forces unless they pulled back from rebel-held areas.
The Russian defence ministry said Turkey had provided artillery support to the militants, wounding four Syrian soldiers.
Russian warplanes then attacked the militants who had burst through the government positions in two areas of Idlib province, allowing the Syrian army to repel them, the ministry said.
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"So as not to allow the armed groups to make it deep into Syrian territory, Russian Su-25 aircraft carried out a strike... on the armed militant groups that burst through," it said.
The Turkish ministry said that Syrian planes had carried out the air strikes.
A rebel source told Reuters that the Turkish army and rebels had mounted a joint operation to storm the town of Nairab, push the army away from the M4 highway, and help relieve the encirclement of five Turkish observation posts on the outskirts of the crossroads town of Saraqeb.
Turkish forces had engaged Syrian troops on Saraqeb's southern edge, he said.
"The strategic goal is to reach Saraqeb city because it lies on both key highways," Ibrahim al Idlibi, a former rebel official and activist said. "The Turkish troops are now combing Nairab town after the Syrian forces had pulled away".
Syrian troops backed by Russian forces have been battling since December to eradicate the last rebel strongholds in northwest Syria in what could be one of the final chapters of a war that has killed an estimated 400,000 Syrians and left much of the country and its cities in ruins.
Turkish and Russian officials had failed to reach any compromise in talks in Ankara, although Turkish officials had sounded more optimistic on Thursday prior to the flare-up on the ground.
Various options were being discussed, including the possibility of joint patrols in the area, one official said. Both Ankara and Moscow expected their presidents to "end the issue," he added.
Syrian President Bashar al-Assad, whose family dynasty has ruled for nearly half a century, has showed no sign of letting up in the campaign to crush his foes once and for all.
Residents and relief staff said Russian warplanes on Thursday resumed attacks on the towns of Darat Izza and Atareb in the northern corner of Aleppo province, where Turkish troops have set up a line of defence.
NOWHERE TO GO
Meanwhile no end was in sight to the misery of the nearly one million people - most of them women and children - who have fled the fighting to seek sanctuary in the border area.
The exodus has overwhelmed relief agencies but Turkey, which is struggling to cope with the 3.7 million Syrian refugees already camped inside its borders, says it can take no more.
In Azaz, about 30 km (20 miles) northwest of Aleppo city, Abu Abdallah had been stranded on the road for days. After his family fled the air strikes pounding Idlib, they moved from one village to another but have yet to find refuge. With him were his wife, four children and 20 other relatives.
"I don't know where to take them," the 49-year-old farmer said, sitting on his tractor. "God knows where we will go."
Families are sleeping outside by roads and in olive groves, burning garbage to stay warm. Some children have died from the cold. Some have already been displaced more than once after fleeing battles in other parts of Syria earlier in the conflict.
At a displaced persons camp in Azaz, people crowded into tents described an exhausting passage to reach what they hoped would be safety.
Raqia Omri, 41, originally from the Aleppo countryside, said she would go with her family to Turkey if she had the chance.
"We want the world so see the pain that we and our children are living with in the tents. The situation is very hard and we died in order to get to here. Today we are women sitting in the street and washing our clothes. We want Turkey to help us."
The U.N. humanitarian affairs agency (OCHA) said the crisis had reached a horrifying level, with more and more displaced people crammed into a small pocket of Idlib.
"Places previously considered safe by civilians are now coming under fire," it said in a report.
More than 300 people, many of them women and children, had been killed in Idlib and Aleppo due to the hostilities since the start of the year, it said. Indiscriminate attacks had damaged or destroyed hospitals, schools and other civilian sites.
The United Nations High Commissioner for Refugees, Filippo Grandi, called for a cessation of hostilities and appealed for urgent action to help those displaced.
"Thousands of innocent people cannot pay the price of a divided international community, whose inability to find a solution to this crisis is going to be a grave stain on our collective international conscience," he said.