The United States has warned Turkey not to get involved in the Nagorno-Karabakh conflict after reports Ankara is sending Syrian fighters to back Azerbaijan. Syrian fighters have said that Turkey is sending rebels to fight for Azerbaijan as it battles Armenia over the breakaway province of Nagorno-Karabakh. Fighting erupted last Sunday, killing as many as 200 and threatening a wider conflict. "We [already] saw Syrian fighters taken from the battlefields in Syria to Libya" and that it "created more instability, more turbulence, more conflict, more fighting, less peace", US Secretary of State Mike Pompeo said as he returned from Croatia. "I think it would do the same thing in the conflict in and around Nagorno-Karabakh as well. So, I hope that reporting proves inaccurate. "This is a long-standing conflict in this border space, when those tensions rise, internationalising this, third parties bringing ammunitions, weapon systems. you increase the complexity, you increase the risk of loss of lives, you decrease the capacity for peace." As in Libya, he said, "we've urged everyone to just stay out of this other than to urge that there be a ceasefire and that dialogue be the methodology by which order is restored, peace is restored. At least we hope that's the case. "We've certainly communicated that to both the Azerbaijanis and Armenian leaders, and to the Turks as well," he added. In a call with his Iranian counterpart on Friday, Russian Foreign Minister Sergey Lavrov expressed concern about the Turkish-funded fighters in Syria and Libya joining the front lines in Nagorno-Karabakh. Iran separately warned against any "intrusion" by Armenian and Azerbaijani forces after mortar fire hit Iranian villages along the border. "Any intrusion upon our country's territory by either side of the conflict is intolerable [and] we seriously warn all sides to take the necessary precautions in this regard," foreign ministry spokesman Saeed Khatibzadeh said in a statement. Clashes have raged between Armenian and Azerbaijani forces since Sunday over Nagorno-Karabakh, an ethnic Armenian province that broke away from Baku in a bitterly fought war in the early 1990s that claimed 30,000 lives. Armenia said on Saturday that 51 more separatist soldiers had died in clashes with Azerbaijani forces as fighting entered a seventh day. The government published a list with the names of the 51 dead servicemen on its website, hours after the leader of the separatist region, Arayik Harutyunyan, said a "final battle" was under way with Azerbaijani forces and that he was joining the fighting on the Karabakh front-line. Nagorno-Karabakh officials have said more than 150 servicemen on their side have been killed so far. Azerbaijani authorities haven’t given details on their military casualties but said 19 civilians have been killed and 55 more wounded. A French attempt to relaunch peace talks over Nagorno-Karabakh showed no sign of a breakthrough on Saturday. French President Emmanuel Macron spoke on Friday with President Ilham Aliyev of Azerbaijan and Prime Minister Nikol Pashinyan of Armenia – which backs Nagorno-Karabakh – and said later in a statement he had proposed a new way to restart talks. Armenia had said on Friday it was willing to engage with Russia, the United States and France – co-chairs of the so-called Minsk Group of the OSCE security organisation – on renewing a ceasefire in Nagorno-Karabakh. But Mr Aliyev said that the Minsk Group had failed for the past three decades to make progress over the dispute. "The president of Azerbaijan placed the entire responsibility on the leadership of Armenia for the break-off of negotiations and the armed confrontation," Mr Aliyev's press service said. Armenia says it was Azerbaijan that reopened the conflict by launching a major offensive on September 27. Mr Aliyev said Azerbaijan was not ignoring ceasefire calls, but this could only be achieved if ethnic Armenian forces withdrew from Azeri territories – a reference to Nagorno-Karabakh and seven surrounding regions they have controlled since the 1990s. "(The) conditions must be that they withdraw from the territories. We need our territories back by peaceful means and we demonstrated for 28 years our willingness to have a peaceful settlement," Mr Aliyev said.