* Rebels shoot down Syrian military helicopter
* Syrian government troops seize key highway
* Turkey-Russia talks in Ankara end inconclusively
* US Syria envoy sets talks to seek end to fighting (Recasts with reported Syrian death toll, other detail)
By Suleiman Al-Khalidi and Orhan Coskun
AMMAN/ANKARA, Feb 11 (Reuters) - Turkey said on Tuesday 51 Syrian soldiers were killed in northwest Syria as Turkish-backed rebels struck back against Russian-supported government forces who had made gains in their campaign to eliminate the last insurgent bastion in the country.
The Turkish Defence Ministry cited sources on the ground for the information, adding that two Syrian tanks and one ammunition store were destroyed as well.
Hours before, a war monitor reported that Syrian government forces seized control of the main Aleppo-to-Damascus highway running through the embattled northwest province of Idlib for the first time since the early days of the civil war in 2012.
But Syrian state media made no mention of this and rebel sources later said fighting was continuing in some northern areas near the M-5 highway, which links Aleppo with the capital Damascus and ultimately Deraa in the far south.
In response, insurgents shot down a Syrian military helicopter and advanced toward the town of Nairab, which the Turkish Defence Ministry said had been abandoned by Syrian government forces.
A Turkish official said the rebels, bolstered by Turkish artillery, had begun "a full-fledged attack" on an area recently lost to the government side near Saraqeb, a strategic crossroads town on the M5 highway. A rebel commander told Reuters they were pushing back government forces there.
For its part, the Syrian army said on Tuesday it would respond to attacks by Turkish forces who it said were trying to halt army advances into Idlib province.
The flare-up of fighting has given rise to some of the most serious confrontations between Ankara and Damascus in the nine-year-old war in which Russia and Iran have backed Syrian President Bashar al-Assad.
U.S.-TURKISH TALKS ON WEDNESDAY
James Jeffrey, the U.S. envoy for Syria, is set to meet senior Turkish officials in Ankara on Wednesday and the U.S. Embassy there said they would discuss working together toward a political solution to the conflict.
"Today, our NATO ally Turkey is facing a threat from Assad's government and Russia. We are here to assess the situation with the Turkish government and offer support if possible," said Jeffrey, who arrived in Ankara late on Tuesday.
Idlib's fate may well be decided by Turkey and Russia as much as by Assad.
Russia has officers on the ground advising the Syrians on the campaign as well as some ground forces, and Russian warplanes have carried out numerous air strikes.
Ankara has sent thousands of soldiers across the border to help stem the Syrian offensive.
Relief agencies meanwhile said an exodus of hundreds of thousands of civilians from the afflicted areas was the largest such movement in the war and marked a new humanitarian crisis.
Turkey already hosts 3.6 million Syrian refugees and says it cannot absorb any more. It said it would halt any new refugee waves from Idlib and its military would remain deployed there.
Turkish President Tayyip Erdogan said on Tuesday the Syrian government would pay a "very heavy price" for attacking Turkish troops, including five killed on Monday and eight Turkish personnel killed a week earlier.
"We gave the necessary responses to the Syrian side at the highest level. Especially in Idlib, they got what they deserved. But this is not enough, it will continue," he said in Ankara, adding he would announce on Wednesday a detailed plan for Idlib.
Talks in Ankara between Turkey and a Russian delegation ended on Monday without agreement on halting the fighting, a Turkish diplomatic source said.
Turkish officials told the Russians that attacks on Turkish posts inside Idlib must cease immediately and that Turkish forces had destroyed several Syrian government targets in retaliation. Erdogan has warned Turkey will drive back Assad's forces unless they withdraw by the end of this month.
The Kremlin said on Tuesday all attacks on Russian and Syrian forces in Idlib must stop.
Russian President Vladimir Putin will discuss the situation with Erdogan by phone later on Tuesday, TASS news agency said.
Since launching their new offensive in December, government forces have recaptured more than 600 square km (230 square miles) of territory, and in recent days wrested control of dozens of towns and villages.
The rebels are a mix of nationalist factions and Islamist militants who have had deadly rivalries but are now closing ranks.
Last week government troops recaptured rebel-held Saraqeb, where Turkey had several military personnel stationed.
Rescue teams said on Tuesday Russian and Syrian warplanes had bombed several towns in Idlib and carried out air raids in nearby western Aleppo province where rebels are present. At least 13 civilians were killed overnight in the air strikes, they said.
The rapid advances by Assad's forces in Idlib have driven nearly 700,000 people - mostly women and children - from their homes towards the sealed Turkish border in the past 10 weeks.
"This is, from our initial analysis, the largest number of people in a single period since the Syrian crisis began almost nine years ago," Jens Laerke, spokesman for the United Nations' OCHA humanitarian agency, told reporters in Geneva.
United Nations High Commissioner for Refugees spokesman Andrej Mahecic said the harsh winter weather was making their suffering worse and that shelter was hard to find.
"Even finding a place in an unfinished building is becoming nearly impossible," he said, adding that mosques were full.
Witnesses and rebels said a new column of Turkish reinforcements, including tanks, rocket launchers and armoured vehicles, crossed the border into Idlib overnight.
The battle for Idlib is a crucial stage of a war that has killed hundreds of thousands of combatants and civilians, made millions refugees in their own country or overseas, and fractured the wider Middle East since it broke out amid the Arab Spring in 2011.
Moscow's military intervention in 2015 helped swing the war decisively in favour of Assad, Syria's ruler for nearly 20 years, but he now presides over a devastated country.
(Additional reporting by Tom Perry in Beirut and Tuvan Gumrukcu in Ankara; Writing by Jonathan Spicer; Editing by Mark Heinrich)
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