The United States and Russia reached a breakthrough deal early on Saturday to try to restore peace in Syria, but air strikes hours later added to rebels' doubts that any ceasefire could hold.

The agreement, by the powers that back opposing sides in the five-year-old war, promises a nationwide truce from sundown on Monday, improved access for humanitarian aid and joint military targeting of hardline Islamist groups.

But hours later, jets bombed a marketplace in rebel-held Idlib in northwestern Syria, killing at least 25 people and wounding dozens, according to locals and rescue workers who said they believed the planes to be Russian.

Idlib province has endured escalating strikes by Russian jets in recent months, according international aid workers and residents, destroying scores of hospitals, bakeries and other infrastructure across rebel-held territory.

Aleppo was also hit from the air and fighting continued on the ground. The army attacked rebel-held areas, both sides said, pushing to maximise gains before the ceasefire deadline.

Ten people were killed by barrel bombs dropped by army helicopters on the besieged rebel-held east of the city, and jets, either Syrian or Russian, bombed rebel-held towns in the northern countryside along important insurgent supply routes, the Britain-based Syrian Observatory for Human Rights said.

Insurgents said they were planning a counter-offensive.

"The fighting is flaring on all the fronts of southern Aleppo," rebel spokesman Captain Abdul Salam Abdul Razak said.

Razak, of the Nour al-Din al Zinki Brigades, part of the Free Syrian Army (FSA) which is backed by the West, said they were studying the peace deal but feared it merely gave the Syrian army a chance to gather forces and pour more Iranian-backed militias into Aleppo.

President Bashar al Assad's government made no comment on the peace deal, but Syrian media were broadly positive.

Syria's mainstream political opposition, the Riyadh-based High Negotiations Committee (HNC), said it had not received a copy of the deal and would only react after consulting members.

A spokeswoman had earlier welcomed any deal that spared civilian lives but cast doubt on whether Moscow would be able to pressure Damascus to stop indiscriminate bombing.

US Secretary of State John Kerry called on all sides to respect the deal, which was finally reached after several failed attempts over recent weeks.

"This requires halting all attacks, including aerial bombardments, and any attempts to gain additional territory at the expense of the parties to the cessation. It requires unimpeded and sustained humanitarian access to all of the besieged and hard-to-reach areas including Aleppo," he said.

Russian Foreign Minister Sergei Lavrov said that despite continuing mistrust, the two sides had developed five documents that would enable coordination of the fight against terrorism and a revival of Syria's failed truce.

Both sides agreed not to release the documents publicly.

"This all creates the necessary conditions for resumption of the political process, which has been stalling for a long time," Lavrov said.

Previous peace efforts have crumbled within weeks, with the United States accusing Assad's forces of attacking opposition groups and civilians.

For many FSA rebels, the idea of a clear separation from Nusra is problematic because on several fronts they fight together against the army and allied Iranian-backed militias.

Fateh Al Sham has also played a major role in trying to end the siege of eastern Aleppo which many rebels say has boosted its popularity, and discussions are ongoing to possibly unify ranks under a broader opposition army.

"Fateh al-Sham is a faction present on the ground and it takes part in most of the military operations, and the matter of separating it is not possible, particularly given that there are attempts to merge, within some factions, with Fateh al-Sham," stated Fares Al Bayoush, head of an FSA group called the Northern Division.-Reuters