BEIRUT/NEW YORK (REUTERS) - Syria announced a new offensive against rebel-held areas of Aleppo on Thursday (Sept 22) while diplomats failed to find a way in New York to revive a US and Russian-brokered ceasefire that collapsed this week.

Warplanes mounted the heaviest air strikes in months against rebel-held districts of Syria's commercial hub and largest city, dealing a fresh blow to efforts to end Syrian civil war that has raged since 2011.

Rebel officials and rescue workers said incendiary bombs were among the weapons that rained down on Aleppo. Mr Hamza al-Khatib, the director of a hospital in the rebel-held east, told Reuters that 45 people were killed.

"It's as if the planes are trying to compensate for all the days they didn't drop bombs" during the ceasefire, Mr Ammar al-Selmo, head of the civil defence rescue service in opposition-held eastern Aleppo, told Reuters.

Moscow and Washington announced the ceasefire with fanfare on Sept 9. But the agreement, possibly the final bid for a breakthrough on Syria before President Barack Obama leaves office in January, collapsed like all previous peace efforts in a 5.5-year-old war that has killed hundreds of thousands of Syrians and made half the nation homeless.

Syrian state media announced the new offensive and quoted the army's military headquarters in Aleppo urging civilians in eastern parts of the city to avoid areas where "terrorists" were located and said it had prepared exit points for those who want to flee, including rebels.

The Syrian army announcement did not say whether the campaign would also include a ground incursion.

The aerial assault, by aircraft from the Syrian government, its Russian allies or both, signalled Moscow and Damascus had rejected a plea by US Secretary of State John Kerry to halt flights so that aid could be delivered and the ceasefire salvaged.

In a tense televised exchange with Russian Foreign Minister Sergei Lavrov at the United Nations on Wednesday, Mr Kerry said stopping the bombardment was the last chance to find a way "out of the carnage".

Syrian President Bashar al-Assad indicated he saw no quick end to the war, telling the Associated Press it would "drag on"as long as it is part of a global conflict in which terrorists are backed by Saudi Arabia, Qatar, Turkey and the United States.

HEAVIEST STRIKES FOR MONTHS

Had the US-Russian brokered truce, which took effect on Sept 12, held and had humanitarian aid consistently flowed to Syria, this could have led to intelligence-sharing by Moscow and Washington to go after Syrian militant groups they both oppose.

The ceasefire deal suffered two blows in the last week. On Saturday, the US-led coalition against the Islamic State in Iraq and Syria (ISIS) militant group carried out a lethal air raid on Syrian government troops.

Washington said it hit Syrian forces by mistake. Assad said in his interview he believed the strikes, which he said lasted over an hour, were deliberate.

On Monday, the ceasefire foundered further with an attack on an aid convoy that killed around 20 people and that Washington blamed on Russian planes. Russia denied involvement.

In another sign of the Syrian government's determination to gain territory, it evacuated more rebel fighters from the last opposition-held district of Homs, which would complete the government's recapture of the central city, now largely in ruins.

LONG, PAINFUL, DISAPPOINTING MEETING

Foreign ministers emerged from a meeting in New York having failed to find a way back to a ceasefire, though the United State's Kerry said he was willing to keep trying if Russia came back with new ideas.

"It was a long, painful, difficult and disappointing meeting," the UN Syria envoy Staffande Mistura told reporters after the meeting of the International Syria Support Group, which includes about two dozen major and regional powers.

Mr Assad, helped by Russian air power and Iranian-backed militias, has steadily tightened his grip on the opposition-held eastern areas of Aleppo this year, achieving a long-held goal of fully encircling it this summer.

Capturing the rebel-held half of Syria's largest city would be the biggest victory of the war for the government side, which has already achieved its strongest position in years thanks to Russian and Iranian support.

The United Nations announced that it was resuming aid deliveries to rebel-held areas on Thursday following a 48-hour suspension to review security guarantees after Monday's attack on the aid convoy near Aleppo.

Mr Assad has appeared as uncompromising as ever in recent weeks, reiterating his goal of taking back the whole country.

The government's main focus has been to consolidate its grip over the main cities of western Syria and the coastal region that is the ancestral homeland of Mr Assad's Alawite sect.