Russia said on Wednesday that it strongly backs the actions of the Syrian leadership to counter an offensive by what it said were terrorist groups receiving support, including drones and training, from outside the country.
The rebels have staged their biggest advance in years over the past week, first seizing Aleppo and now battling government forces and allied militia near Hama, another major city.
"We strongly condemn this attack ... There is no doubt that they would not have dared to commit such an audacious act without the instigation and comprehensive support of external forces that seek to provoke a new round of armed confrontation in Syria, unfurl a the spiral of violence," Russian Foreign Ministry spokeswoman Maria Zakharova told reporters.
Russia is a key ally of Syrian President Bashar al-Assad and has provided him with military support since 2015 in the country's civil war. It has intensified air strikes on rebel targets in response to the latest offensive, according to military sources.
"We express solidarity with the leadership of Syria," Zakharova said. "We strongly support the efforts of the Syrian authorities to counter terrorist groups and restore constitutional order."
Zakharova said, without providing evidence, that the rebels -- including some from former Soviet countries -- had received drones from Ukraine and training in how to operate them.
Ukraine's foreign ministry said it "categorically" rejected that accusation.
It said in a statement on the Telegram messaging app that Ukraine, unlike Russia, unconditionally adhered to international law and that Russia and Iran were responsible for the deteriorating security situation in Syria.
Reporting by Dmitry Antonov