The UK-based Syrian Observatory for Human Rights reported 68 air raids and 19 barrel bombs dropped on several towns and villages in Idlib and Hama provinces in less than three hours on Saturday.

The air raids in the densely populated rebel-held area were the "most intense" in weeks and followed Russian airstrikes that killed four rebels and a shepherd in Idlib province on Friday.

Read more: Germany, Turkey fear escalation in Idlib

The raids targeted jihadist and rebel positions, some of which were empty and others in use, Observatory head Rami Abdel Rahman said. Idlib is largely controlled by Hayat Tahrir al-Sham, an Islamist alliance led by an al-Qaeda-affiliated group.

The bombings included barrel bombs dropped from helicopters into the area of three million people, about half of whom are already displaced from fighting elsewhere in Syria.

More than 350,000 people have been killed and millions displaced since Syria's civil war started in 2011 with the repression of anti-Assad protests.

Damascus' 'retaliation' 

The government said it was retaliating against overnight shelling from rebel-held areas on a government-held town in Hama province, south of Idlib, which killed nine civilians, according to state media. Syrian President Bashar Assad's regime has been sharpening its talk of retaking Idlib in the past month.

The bigger picture

The renewed violence comes after Russia, regime ally Iran and rebel backer Turkey on Friday failed to agree a solution to avert a government offensive. Russia and Iran are key allies of Assad, while Turkey supports some of the rebels.

A summit in Tehran on Friday between Russian President Vladimir Putin, Iranian President Hassan Rouhani and Turkish President Recep Tayyip Erdogan was seen as an opportunity for a diplomatic solution before a full assault on Idlib. The three nations are allied against Islamic State.

Read more: In Syria, rebel defeat marks 'end of revolution'

US toothless

The US has little leverage to stop Russia, Iran and Syria pressing ahead with a massive military assault against Syria's northwest Idlib province.

The new US special envoy for Syria said recently that the US would stay in Syria until the complete eradication of the Islamic State group and will be watching Idlib closely over the next week ahead of UN-led talks on Syria in Geneva on September 14.

Idlib is widely seen as the last opportunity for the US to exert influence in Syria, and if the province falls before the talks, the Trump administration's efforts to re-engage with peace talks could fail.

jbh/rc (AP, AFP, dpa)