US Secretary of State John Kerry said he was working with the White House and Pentagon on how to respond to the Kremlin proposal for military-to-military talks as Moscow prepares its own fight back against the "Islamic State" (IS) militant group.

The suggestion for talks at the military level comes as Russia beefs up its armed forces' presence in Syria, having renewed its commitment to Syrian President Bashar al-Assad and vowing to tackle "terrorism."

The offer came in phone calls with Russian Foreign Minister Sergei Lavrov earlier this week, Kerry told reporters on Wednesday, adding that Moscow wanted to "discuss the issue of precisely what will be done to de-conflict" - ensuring that US and Russian aircraft do not come into conflict with each other.

"They [Russia] say specifically that they are focused on opposing Islamic State forces, and that they're going in to do that," Kerry said. "Now, obviously, there are serious questions about that."

US officials said Moscow has been sending two military cargo flights a day to an air base at Latakia, an Assad stronghold.

Reports suggest a portable air traffic control station, artillery, tanks and other apparatus have been deployed. Some analysts think Russian President Vladimir Putin has ordered the reinforcements to bolster the Assad regime, which has suffered a number of military setbacks in recent weeks.

Conflict could worsen

Washington has reiterated that Russia's military backing for the Syrian government risks escalating the four-and-a-half-year, multi-front conflict, which has claimed more than 240,000 lives.

But Syria's UN envoy questioned why Russians should be forbidden from taking part in air strikes against IS jihadists. "After all, we are fighting the same enemy," Ambassador Bashar Ja'afari told reporters in New York on Wednesday.

Despite a diplomatic offensive by Moscow to include Assad in the fight against the IS militants, Western and regional powers have ruled out working with his regime. Saudi Arabia said the Syrian leader's ouster was just a "matter of time."

On Wednesday, United Nations Secretary-General Ban Ki-moon indirectly criticized Russia, suggesting that "parties arming and providing arms" would only make the situation in Syria "worse." Ban called for a "political solution" to the conflict, appearing to reaffirm an international call for Assad to step down.

Assad blames West

Meanwhile, Assad chided Western nations over the escalating refugee crisis in Europe, claiming that the West has been supporting "terrorists" since the beginning of the crisis and is now "crying" over refugees.

"If you are worried about them, stop supporting terrorists," he said in an interview with Russian media on Wednesday.

As Washington prepared its response to Moscow's request for talks, the Kremlin denied a claim that Russia had offered to making Assad quit "in an elegant way."

Former Finnish president Martti Ahtisaari told the British newspaper "The Guardian" that the comments were made by a senior Russian negotiator in 2012.

mm/sms (AFP, AP, dpa, Reuters)