Airstrikes hit the only road into rebel-held areas of Aleppo city on Sunday in the heaviest bombing since February, a rebel official and monitors said, jeopardizing access where around 300,000 Syrians live.

Russian warplanes carried out the attacks on the Castello road, which was still open but dangerous, the official and monitors said. Defense officials from Syria's government and its ally Russia could not immediately be reached for comment.

The Syrian Observatory for Human Rights group said the road was hit in a week of escalating airstrikes, with Sunday's attack the most intense yet.

The city of Aleppo, about 30 miles south of the Turkish border, is divided between the government and rebels fighting to topple Syrian President Bashar al-Assad.

A truce was brokered by the United States and Russia in February. But the agreement has since unraveled, with fighting and bombardment in Aleppo playing a big part in its collapse.

Kurdish-led YPG forces, which control the Sheikh Maqsoud area in Aleppo that overlooks the Castello road and are tacitly aligned with the government, have also disrupted the road with snipers who target civilians using the road that is a lifeline for the city to the countryside.

Mainstream Syrian rebel groups said on Sunday they would no longer abide by the U.N. truce deal unless the Syrian army ended a major assault on their positions in the suburbs of Damascus within 48 hours.

A statement by the Free Syrian Army (FSA) signed by nearly 40 rebel groups that operate across Syria said they would deem the cessation of hostilities deal as having "totally collapsed" if the assault by Syrian government and allied Lebanese Hezbollah forces fighters did not cease within two days. 

The signatories, who include Western- and Turkish-backed groups operating on the main frontlines in northern and southern Syria, said that once the two-day period had ended, rebels would respond with "all the legitimate means to defend the civilians living in these areas".

In their joint statement, the rebels said the continued attacks by the army on the besieged rebel-held areas around Damascus and their strongholds in Aleppo city and Idlib province were putting peace-making efforts at risk.

The Syrian army stopped extending the cessation of hostilities this month after accusing rebels of violating the agreement by firing at government-controlled residential areas

Russian Jets Pound Aleppo Highway

A senior official in the rebel group Fastaqim that operates in the Aleppo area said there was heavy Russian bombing of rebel areas on the strategic route all day, developments the Syrian Observatory confirmed later.

"From 1 a.m. until 10 a.m., Russian jets were not quiet on the Handarat-Castello front," said Zakaria Malahefji. "A group [of fighters] stationed there was killed."

Observatory Director Rami Abdulrahman said the raids had been happening for a week. "This is more intense than the last days," he added.

A Russian defense ministry statement issued on Saturday accused insurgents of firing missiles at nearby areas, identifying them as members of the al Qaeda-linked Nusra Front, which was not included in the truce.

Rebels fighting under the banner of the Free Syrian Army say Nusra has little or no presence in Aleppo city.

Russia deployed its air force to Syria last year to bomb in support of the Syrian military and its allies.

Rebels say they can distinguish Russian from Syrian warplanes by the accuracy and intensity of their bombing, the way they fly in squadrons and the fact they carry out raids at night.

Syrian government forces have mounted several offensives aimed at encircling rebel-held eastern Aleppo but these have all failed to date. West of Palmyra city, which the Syria army took with Russian aid, heavy battles continue to rage with militants.

Rebels said air strikes on residential areas and busy market places on rebel-held towns and cities across northern Syria that killed dozens would not go without being avenged.

On Sunday, several civilians were killed in aerial bombing of rebel held Idlib city's main busy market place.