Russia says the United States is "unprepared" for including the Syrian government in international plans to counter extremist Islamic State (IS) militants.

Speaking on Russian television on August 9, Foreign Minister Sergei Lavrov said he discussed President Vladimir Putin's proposal on coordinating efforts against IS during a recent meeting in Qatar with U.S. Secretary of State John Kerry.

"I asked him if it wouldn't be easier to reach an agreement, to sit down at the negotiating table and discuss everything with the involvement of the Syrian Army," Lavrov said. "Unfortunately, the Americans are unprepared for that."

Lavrov said he also expressed concern about a U.S. decision to provide air support for Syrian opposition groups fighting against the government of Syrian President Bashar al-Assad.

He said that he warned Kerry that some "grave mistake," such as the accidental bombing of civilians, could "explode the situation to such an extent that nobody would be able to control it."

Lavrov also rejected recent allegations that the Assad government has undeclared stockpiles of chemical weapons. He said "groundless accusations" must be avoided and gave his "guarantee" that all the claims "will be double-checked."

"We have every reason to believe that the Syrian government will continue cooperating closely," Lavrov said.

Based on reporting by TASS and Interfax