Russia's UN ambassador has accused Ukrainian intelligence services of helping rebels fighting the government of Syrian President Bashar al-Assad.

Rebels fighting the Islamist group Hayat Tahrir al-Sham (HTS) "not only did not hide the fact that they are supported by Ukraine, they openly boast about it," Vasiliy Nebenzia told the Security Council.

The ambassador said there was an "identifiable trace" showing that Ukraine's GUR military intelligence service was providing weapons to fighters in northwest Syria.

"Ukrainian military instructors from the GRU are present ... to train HTS fighters for combat operations," including against Russian troops in Syria, Nebenzia said.

A renewed offensive in the past week, led by the HTS and its allies, has seen a massive shift in the long-frozen fronts of Syria's civil war, with rebels advancing on Syria's fourth-largest city of Hama after capturing Aleppo.