MOSCOW — A top Russian diplomat has warned Syrian President Bashar Assad over a statement he made promising to retake the whole country, threatening a withdrawal of Russian support if Assad does not come back into line with the policies Moscow is pursuing in the region.

In a rare sign of disagreement between Moscow and Damascus, Russia's envoy to the United Nations Vitaly Churkin complained Assad did not appear to be sufficiently grateful for Russia’s backing.

“They should follow our advice,” Churkin said in an interview with Russia’s Kommersant newspaper published Thursday.

“Russia has made serious political and diplomatic, and now military, investments in this crisis. Therefore, of course, we would like Assad to respond,” he said.

Moscow has closely backed Assad since the beginning of Syria’s five-year civil war and in September launched a bombing campaign that reversed the Syrian president's fortunes. With close Russian air support, Syrian troops now appear poised to seize the key northern city of Aleppo.

Military success appears to have made Assad more belligerent, and in a televised interview earlier this week he said his forces would not stop fighting until they had seized back the entire country, most of which is controlled by myriad rebel groups, including the so-called Islamic State group.

Assad also said a ceasefire brokered by the U.S. and Russia, which is due to come into force Friday, would not mean his soldiers would lay down arms.

Assertiveness from Assad has gone down badly in Moscow and Churkin took the opportunity to remind the Syrian president the success of his forces had been facilitated by Russian airstrikes and other military assistance that could be withdrawn at any time.

“The effective operation by Russia’s air force has allowed [the Syrian army] to drive back its enemies from Damascus. Now [government forces] might take Aleppo, the biggest urban center in the north of the country. But if the default position of [the Syrian authorities] is that no truce is necessary and that they need to fight to the victorious end, then this conflict will continue for a very, very long time. And that’s terrible to imagine,” Churkin said, Kommersant reported.

Russian and U.S. military officials held talks in Geneva Friday in an attempt to achieve a cessation of hostilities in Syria, the Reuters news agency reported.

A meeting of the larger International Syria Support Group is expected to take place at the United Nations in Geneva later Friday.