Ukraine says Syrian collapse shows Russia can't fight on two fronts

The collapse of Russia's ally Syria in the face of an assault from rebel groups shows Moscow cannot fight on two fronts, Ukraine's foreign ministry said on Thursday while reiterating denials that Kyiv was involved in the fighting there.

"We can see that Russia cannot fight on two fronts -- this is clear from the events in Syria," ministry spokesperson Heorhii Tykhyi told reporters at a weekly news conference.

After years locked behind frozen frontlines, Syrian rebels have burst forth to mount the swiftest battlefield advance by either side since a rebellion against President Bashar al-Assad descended into civil war 13 years ago.

Tykhyi was responding to a question about accusations from Iran, another ally of Assad, that Ukraine was supporting what Tehran called terrorist groups.

"Ukraine categorically and decisively rejects any accusations ... about our supposed involvement in the deteriorating security situation in Syria," the spokesperson said.

Assad's government has been able to hold on to a significant chunk of the country after years of bitter fighting.

However, a week-long lightning offensive by the rebels has seen them capture two major cities and swathes of land.

"Russia's significant losses in Ukraine have led Moscow to withdraw the majority of its troops and equipment from Syria, leaving its ally ... without the necessary support," Tykhyi said.

Russia launched a full-scale invasion of Ukraine nearly three years ago. Tens of thousands of soldiers and Ukrainian civilians have been killed in the ensuing battle.