Russian warplanes hit a key rebel supply route to Aleppo on Sunday in Moscow's first strikes on Syria's battleground second city since a February ceasefire, a monitoring group said.

"The Russian and Syrian warplanes together carried out at least 40 air strikes on the Castello road," Syrian Observatory for Human Rights chief Rami Abdel Rahman said.

"They are the heaviest air strikes there since February, and they are also the first confirmed Russian strikes since the truce began," Abdel Rahman said.

The violence wracking Aleppo city over the past month has killed some 300 civilians and left world powers scrambling to save the fragile truce brokered by the United States and Russia nearly three months ago.

The northern city -- once Syria's commercial powerhouse -- is divided between rebel groups in the east and regime forces in the west.

The Castello road is a key supply route for rebels leading north out of Aleppo.

Even while the rest of the city witnessed relative calm in the first few weeks of the truce, fierce fighting has raged for the highway.

On Friday, Moscow proposed joint air strikes with Washington against jihadists in Syria from Wednesday, but its offer was spurned.

Russia has been carrying out air strikes in Syria since last September in support of its ally President Bashar al-Assad.

Washington launched its air war against the Islamic State group and other jihadists in Syria in 2014.

Pentagon spokesman Navy Captain Jeff Davis said the US military does "not collaborate or coordinate with the Russians on any operations in Syria."

US State Department spokesman John Kirby said nothing had been agreed with Moscow as its ally Damascus was responsible for the "vast majority" of violations of the February ceasefire.

Russia's intervention has significantly strengthened the Syrian government in the five-year-old civil war that has killed more than 270,000 people and driven millions from their homes.