BEIRUT - 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 militants 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 militants 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.

Meanwhile, bombings suspected to have been carried out by the Islamic State group killed at least eight people in northeastern Syria hours after a top US commander visited, security forces said Sunday.

Washington regards the Kurdish-led militia that controls most of the northeast as the most effective fighting force against IS in Syria and the head of US Central Command General Joseph Votel made a secret visit Saturday to confer with US military advisers working with them.

A CENTCOM spokesman declined to give details of the visit, saying only that Votel had visited several location inside Syria on the highest-ranking visit to the country since the 2011 outbreak of the civil war.

Two IS suicide bombers struck the centre of Qamishli, a mainly Kurdish city that is the de facto capital of the swathes of northern Syria where Kurdish militia have set up a self-declared autonomous administration.

The bombers hit a restaurant and a bakery in the Christian Wusta neighbourhood of the city that is controlled by a breakaway Christian militia that backs the Damascus regime.

A militia spokesman said three Christians were killed and 15 wounded in the bombings.

The IS-affiliated Amaq news agency reported the bombings but issued no claim.

Hours earlier, two car bombs hit a Kurdish checkpoint outside the town of Tal Tamr, about 40 kilometres (25 miles) from the Turkish border.

Kurdish security force spokesman Abdallah Saadoun told AFP that there had been advance warning of the attack but that five security force personnel were killed nonetheless.

Amaq said a "suicide operation" had killed more than a dozen Kurdish security personnel but issued no claim.

Washington has kept up its support for the Kurdish militia in northeastern Syria despite strong opposition from NATO ally Ankara whose misgivings have prevented the delivery of heavier weapons.

Ankara regards the main Syrian Kurdish party as a puppet of the Kurdistan Workers Party (PKK) which has waged a deadly three-decade insurgency in southeastern Turkey.

It has repeatedly shelled Syrian Kurdish positions when the militia has made advances in border territory that Turkish commanders consider sensitive.