The Russian military has fired multiple cruise missiles from its ships in the Mediterranean aimed at the Isis stronghold of Raqqa, according to reports.
US officials were quoted by the Reuters news agency as saying Russia had launched a "significant number" of strikes in Syria using both sea-launched cruise missiles and long-range bombers.
The French daily Le Monde cited senior French government officials as saying Russian missiles had struck Isis positions in Raqqa, its de facto capital in its Syrian territories.
According to reports, Russian officials gave their US counterparts notice ahead of the strikes, in a sign of closer cooperation between the two states after talks between Vladimir Putin and Barack Obama at the G20 summit.
Videos showing alleged cruise missiles flying and crashed in Syria appeared across social media on Tuesday, though their provenance could not be immediately verified.
Russia has vowed to escalate its military campaign in Syria after it was confirmed the airliner which crashed in Sinai was brought down by a bomb.
"Our military work in Syria must not only continue," Mr Putin said. "It must be strengthened in such a way so that the terrorists will understand that retribution is inevitable."
Speaking to the Associated Press, an unnamed US defence official said the Russian military had communicated prior to the attacks with the US-led coalition's centre of operations at the al-Udeid air base in Qatar.
Russia has been launching air strikes on Isis in Syria since the end of September, although it has faced accusations of targeting moderate Syrian rebels in a bid to support regime forces.
