"As you know, we regularly invited representatives of the United States to such meetings as observers. They initially took part, then they decided to leave these invitations unanswered. Although I am sure that [US participation] would certainly be useful," the foreign minister told reporters in Moscow.
Earlier, the official said the next meeting on Syria in Astana would be held in mid-February.
Commenting on the presence of Jabhat al-Nusra terrorist group (banned in Russia) in Syria's province of Idlib, Lavrov said that it violated the Russian-Turkish agreement on security in the region.
"A terrorist nest that is still in Idlib is a fact, and our Syrian colleagues have confirmed their readiness to eliminate this terrorist hotspot. We are ready to continue acting as set out in the Russian-Turkish agreement on Idlib, including the establishment of a demilitarized zone around the security zone. But the fact that Nusra and its reincarnation, Tahrir al-Sham (banned in Russia), have, in fact, taken over a large portion of the territory there, of course, does not correspond to the agreements that were reached on the security issues of Idlib," Lavrov told reporters after talks with his Kazakh counterpart Beibut Atamkulov.
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The Russian foreign minister added that Turkey was working with the Russian military to try and resolve the matter.
Lavrov went on to note that Moscow believed t was possible for Turkey and Syria to rely on the 1998 Adana pact to ensure security of the common border.
"We believe it is possible for Turkey and Syria to use the so-called Adana agreement of 1998. As I understand it, the Syrian government has also made a statement the other day indicating readiness to work on the basis of this agreement to ensure border security," the minister said at talks in Astana, commenting on the situation in Syria's northwest after the United States announced its decision to withdraw troops.
Erdogan said at a joint press conference with Putin on Wednesday that Ankara and Moscow would continue their cooperation in Idlib, where more than 10 different militant groups are currently operating, including the Jabhat al-Nusra terrorist organization (outlawed in Russia).\
*Jabhat al-Nusra [the Nusra Front, al-Nusra] — a terrorist group banned in Russia