Aleppo after a rebel attack, November 30, 2024. (REUTERS)

Russia's Defense Ministry said its air force had carried out strikes on Syrian rebels in support of the country's army, Russian news agencies reported. The strikes followed what was the boldest rebel assault for years in a civil war where front lines had largely been frozen since 2020.

Hayat Tahrir al-Sham, or HTS, once known as the Nusra Front, is designated a terrorist group by the US, Russia, Turkey and other states.

In Washington, the White House National Security Council said it was closely monitoring the situation and had been in contact with regional capitals over the last 48 hours.

NSC spokesperson Sean Savett "urged de-escalation and a serious and credible political process" under the 2015 UN Security Council Resolution 2254, which laid out the steps for a ceasefire and political transition.

The war, which has killed hundreds of thousands of people and displaced many millions, has ground on since 2011 with no formal end, although most major fighting halted years ago after Iran and Russia helped Assad's government win control of most land and all major cities.