Russia slammed United States President Barack Obama's decision to send more troops to Syria, stating that it violated Damascus' sovereignty. Obama had announced earlier this week that he would send 250 Special Operations Forces to Syria to boost the fight against the Islamic State group.

Russian Foreign Minister Sergei Lavrov called the U.S. move a "big mistake," also describing the U.S-led coalitions actions in Syria "illegal."

"I've told our American partners repeatedly that this is a big mistake. Just like they received approval from the Iraqi government, they should have obtained approval from Damascus or come to the UN Security Council," Lavrov told Swedish newspaper Dagens Nyheter, according to Sputnik International

Sergey Ryabkov, the Russian deputy foreign minister, said on Friday that Moscow was concerned about the U.S. action, which it said comes "without coordination with the legitimate government" of Syria, according to Tass, a Russian news agency.

"If we are talking from the position of the foreign ministry, we, of course, cannot not be concerned over the fact that US carries out such actions without coordination with the legitimate government of the Syrian Arab Republic," Ryabkov said. "This is a violation of sovereignty."

The statement comes on the day reports said the Russian Air Force would aid the Syrian army in carrying out offensives in the ISIS-controlled cities of Deir al-Zor and Raqqa. Russia has been carrying out airstrikes in Syria since September last year on the request of Syrian President Bashar al-Assad. However, last month, Russian President Valdimir Putin announced that the Russian forces would start pulling out of the country, though officials had said airstrikes against ISIS, the Nusra Front and rebel groups in Syria would continue.

The fresh differences between the U.S. and Russia come even as the ceasefire in Syria hangs by a thread, especially after this week's bombing of a hospital in the city of Aleppo. The U.S. State Department on Thursday urged Russia to use its "influence" over Assad to stop such attacks on civilians, Reuters reported.

On Thursday, the United Nations special envoy for Syria, Staffan de Mistura called on Russia and the U.S. to "revitalise" the Syrian ceasefire.