* Turkey, Russia agreed in 2018 on de-escalation zone
* Syrian government offensive upsets the fragile pact
* Ankara threatens to strike back against Syrian forces
* Syrian forces control key highway in the region (Adds Syrian army movements, further reinforcements, further Cavusoglu comments)
By Tuvan Gumrukcu
ANKARA, Feb 15 (Reuters) - Turkey said on Saturday it had fulfilled its responsibilities in Syria's Idlib region in line with de-escalation agreements with Russia and Iran, warning it would take military action in the area if diplomatic efforts with Moscow fail.
Turkey and Russia, which back opposing sides in Syria's war, agreed in 2018 to set up a de-escalation zone in the northwestern region. But their fragile cooperation has been disrupted by a Syrian government offensive in Idlib, in which 13 Turkish soldiers have been killed in the past two weeks.
Ankara has said it will use military power to drive back the Syrian forces unless they withdraw by the end of February, and President Tayyip Erdogan threatened to strike Syrian government forces anywhere in Syria if another Turkish soldier was hurt.
Russia, which backs Syrian President Bashar al-Assad, says Turkey has flouted deals it made with Moscow and aggravated the situation in Idlib. The Kremlin also said Ankara had failed to neutralise militants there.
Turkish Vice President Fuat Oktay told broadcaster NTV that Turkey was determined to stop Syrian advances in Idlib, and that Ankara had conveyed its position to Moscow during ongoing talks.
"We cannot overlook the cruelty happening in our neighbour," Oktay said. "Turkey has fulfilled its responsibilities in Idlib. Some of our observation posts have fallen into areas controlled by the (Syrian) regime," he said, referring to Turkish military observation posts established in Idlib under the 2018 deal.
The state-run Anadolu news agency said on Saturday buses carrying commandoes and trucks loaded with more military equipment had been sent to reinforce the Turkish observation posts.
In Russia, news agencies quoted a military source as saying rebels in Idlib had received U.S.-made portable air defence systems from Turkey, and that Ankara had sent more than 70 tanks, some 200 armoured vehicles and 80 artillery weapons to the region.
Speaking during an international security conference in Munich, Turkish Foreign Minister Mevlut Cavusoglu said Ankara wanted to resolve matters with Russia over Idlib through diplomacy, but would take other steps if necessary.
"If it won't work through diplomatic channels, we will take the necessary steps," Cavusoglu said, adding that a Turkish delegation would go to Moscow on Monday for talks and that he would meet his Russian counterpart later in the day.
Cavusoglu added later that Turkey expected its allies to pressure Syria's government and its guarantors, Russia and Iran, to halt attacks in Idlib and push for a political solution.
ESCALATION OF VIOLENCE
The escalation of violence in Idlib has also caused hundreds of thousands of people to abandon their homes and head north to the Turkish-Syrian border, many trudging by foot through snow in freezing temperatures, to escape air strikes and artillery fires by the Russian-supported government forces.
Turkey, which currently hosts 3.6 million Syrian refugees, has said it cannot handle a new influx from Idlib. It has poured more than 5,000 troops, several convoys of military vehicles and equipment to the region, including tanks, armoured personnel carriers and radar equipment to bolster its positions.
As the Syrian government continued its offensive, Turkish and Russian officials held talks in Ankara to tackle the dispute. Erdogan has also spoken on the phone twice with Russian President Vladimir Putin since the Turkish troops were killed.
Turkey, a NATO ally with the alliance's second-biggest army, has supported rebels looking to oust Assad. Erdogan said earlier this week that the Turkey-backed rebels launched an offensive to retake some areas they had lost to Syrian forces.
However, Syrian state media said on Saturday that Syrian troops began clearing barricades from the key M5 highway between Damascus and Aleppo after recovering full control of the road for the first time in more than seven years in a major gain for Assad.
State media also reported Syrian advances towards rebel-held Atarib, some 24 kilometres (15 miles) west of Aleppo, and three towns to its northwest. (Additonal reporting by Tom Perry in Beirut and Polina Ivanova in Moscow; Editing by Edmund Blair and Helen Popper)
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