ALEPPO -  Missiles rained down on rebel-held areas of Syria's Aleppo on Friday, causing widespread destruction that overwhelmed rescue teams, as the army prepared a ground offensive to retake the city.

Heavy bombardment of rebel-held areas of eastern Aleppo has killed more than 70 people and has destroyed at least 40 buildings since Friday morning, Ammar al Selmo, the head of civil defence there, told Reuters.

The intensity of the bombardment, which included artillery barrages and barrel bombings by helicopters, brought new misery to the estimated 250,000 civilians besieged by the army.

The escalation came after US Secretary of State John Kerry failed to reach an agreement with Russian counterpart Sergei Lavrov on Thursday on terms to salvage a failed ceasefire. The two met again on Friday at the United Nations and made what Kerry said was "a little bit of progress" on resolving their differences on Syria. "We're evaluating some mutual ideas in a constructive way, period," Kerry told reporters.

Asked at the UN earlier whether the truce could be reinstated, Lavrov simply said: "You should ask the Americans."

He later told the UN General Assembly that US-Russian agreements aimed at ending the Syria conflict must be salvaged, saying there was "no alternative" to the process. "Now it is essential to prevent a disruption of these agreements," Lavrov said.

Sergei Lavrov told the United Nations on Friday that US-Russian agreements aimed at ending the war in Syria must be salvaged as fighting raged on the ground.

Lavrov said there was "no alternative" to the Russian and US-led peace process and insisted that "now it is essential to prevent a disruption of these agreements."The foreign minister addressed the General Assembly after holding talks with US Secretary of State John Kerry on reviving a ceasefire in Syria that was shattered this week.

The Syrian army declared an end to the truce on Monday following a US-led coalition strike on Syrian soldiers near Deir Ezzor that Washington said was unintentional.

Thursday's Kerry-Lavrov talks in New York broke up after Russia refused US demands that it promise to immediately ground the Syrian regime's air force.

Also in New York, French Foreign Minister Jean-Marc Ayrault accused Syrian President Bashar al-Assad of "playing the card of a partition" of his country with the Aleppo offensive.

An AFP journalist in rebel-held east Aleppo reported relentless air raids and artillery fire overnight and Friday morning. Entire apartment blocks were flattened, overwhelming rescue teams from the White Helmets civil defence organisation.

In the Al-Kalasseh district, three buildings were levelled by a single strike, and rescue workers tried frantically to reach survivors using a single bulldozer and their bare hands.

The White Helmets' headquarters in the Ansari district was badly damaged along with an ambulance and a fire engine. A second centre operated by the group was also hit.

Rescue workers told AFP their stock of diesel was down to 2,000 litres (530 gallons), forcing them to ration fuel and make choices on when to intervene.

Also in Aleppo province, the Observatory reported 15 deaths including 11 children in a Russian raid on the rebel-held town of Beshkatine and 11 killed in raids by unidentified aircraft on Islamic State group stronghold Al-Bal.

The bombardment came a day after the Syrian army announced an offensive to recapture east Aleppo, which has been held by the rebels since mid-2012 but has been surrounded by government forces since July.

The army urged civilians to distance themselves from "the positions of terrorist groups" and pledged that fleeing residents would not be detained.

A high-ranking military source confirmed that the bombardment was preparation for a ground assault. "We have begun reconnaissance, aerial and artillery bombardment," he told AFP. "This could go on for hours or days before the ground operation starts. The timing of the ground operation will depend on the results of the strikes and the situation on the ground."

The conflict in Syria has cost more than 300,000 lives and displaced over half the country's population since it erupted in March 2011.

In a bid to relaunch peace talks, Kerry and Lavrov announced a ceasefire on September 9, with Moscow responsible for forcing government troops to stand down and allow in UN aid convoys.

Washington was supposed to pressure rebel forces to respect the truce and distance themselves from jihadists, but the ceasefire fell apart acrimoniously and the Syrian army declared it over on Monday.

UN peace envoy Staffan de Mistura said Thursday's failed talks were "long, painful and disappointing" and warned of escalating violence.

In Geneva, the UN said Friday it was considering a different route to send desperately needed aid to east Aleppo to circumvent the blocked main supply route.

In Damascus, an analyst close to the regime said it was no coincidence that the Aleppo assault began as the New York talks broke down.

"In Aleppo, negotiations are being conducted by fire," he said.

"The Americans must understand that so long as they don't implement their commitments, particularly for the rebels to distance themselves from... (jihadists), the Russians and the Syrian army will advance."