BEIRUT (REUTERS) - Syrian rebels and pro-government forces battled each other on major frontlines near Aleppo and Hama, and air strikes reportedly killed a dozen people including four medical workers, as a ceasefire appeared to have completely unravelled.

The renewed battles demonstrated the thin prospects for reviving a truce that collapsed into fresh fighting and bombardments on Monday (Sept 19), including an attack on an aid convoy which US officials believe was carried out by Russian jets. Moscow denies involvement.

The UN Security Council was due to hold a high-level meeting on Syria later on Wednesday.

Despite accusing Moscow of being behind the bombing of the aid convoy, the United States says the ceasefire agreement it sponsored jointly with Russia is "not dead".

But the deal, probably the final hope of reaching a settlement on Syria before the administration of President Barack Obama leaves office, is following the path of all previous peace efforts in Syria: still being touted by diplomats long after the warring parties appeared to have abandoned it.

Overnight fighting was focused in areas that control access to Aleppo city, where the rebel-held east has been encircled by government forces, aided by Russian air power and Iran-backed militias, for all but a few weeks since July.

Syrian state media and a TV station controlled by its Lebanese ally Hezbollah said the army had recaptured a fertiliser factory in the Ramousah area to the south-west of the city. The Syrian Observatory for Human Rights, a monitoring body, confirmed the advance and said government forces had pressed forward near an apartment complex nearby.

A rebel fighter in the Aleppo area said warplanes had been bombing all night in preparation for an attack. But "the regime's attempts to advance failed," said the rebel, speaking to Reuters from the Aleppo area via the internet.

A Syrian military source said insurgent groups were mobilising to the south and west of Aleppo, and in the northern Hama area.

"We will certainly target all these gatherings and mobilisations they are conducting." The army reported carrying out air strikes on seven areas near Aleppo. The Observatory said an air strike killed four medical workers and at least nine rebel fighters in the insurgent-held town of Khan Touman south of Aleppo, saying the rebels were part of the Islamist alliance Jaish al-Fatah.

The medical staff killed were working for the Union of Medical Care and Relief Organizations (UOSSM), it said. UOSSM confirmed in a statement that at least four of its staff had been killed.

Syrian government forces also launched a major advance in Hama province in the West of the country. "It is a very intense attack, for which Russian jets paved the way, but it was repelled by the brothers, praise God," Abu al-Baraa al-Hamawi, a rebel commander fighting as part of the Islamist Jaish al-Fatah alliance, told Reuters.

He said rebels had destroyed four tanks and inflicted heavy losses on government troops. Syrian state TV said government forces had killed a number of insurgents and destroyed their vehicles.

Rebel sources also reported an attempt by pro-government forces to advance in the Handarat area to the north of Aleppo, saying this too had been repelled. Pro-government media made no mention of that attack.

The Observatory reported that a Syrian jet had crashed near Damascus, saying the cause of the crash and fate of the pilot were unknown. ISIS said it had been shot down.

"BAR OF DEPRAVITY"

The truce brokered by the United States and Russia took effect on Sept 12 as part of a deal meant to facilitate aid access to besieged areas.

Foreign ministers of 20 countries including the United States and Russia met to discuss it on Tuesday and gave the agreement their support. US Secretary of State John Kerry said after the meeting: "The ceasefire is not dead".

In the pact, the details of which remain secret, Washington and Moscow, which back opposing sides in the war between President Bashar al-Assad's government and insurgents, agreed to jointly target militants that are their common enemy.

But such unprecedented cooperation, at a time when trust between the Cold War-era foes is at its lowest for decades, was always a risky gamble. Kerry agreed the deal despite scepticism among other senior US administration figures, and has acknowledged that it is fragile and uncertain.

Tensions between the United States and Russia escalated over a Sept 17 attack by the US-led coalition against ISIS that killed dozens of Syrian soldiers in the eastern Deir al-Zor province. Washington said that strike was carried out by mistake with the intent of hitting ISIS.

Monday's attack on the aid convoy, which the Syrian Red Crescent says killed the head of its local office and around 20 other people, brought furious international condemnation.

The United Nations suspended aid shipments. Secretary General Ban Ki Moon used his farewell speech to the General Assembly in New York to denounce the "cowards" behind it. "Just when you think it cannot get any worse, the bar of depravity sinks lower," Ban said.

However, the United Nations, which initially described the attack as an air strike, rowed back from that characterisation, saying it could not be certain what had happened.

Two US officials told Reuters on Tuesday that two Russian Sukhoi SU-24 warplanes were in the skies above the aid convoy at the time it was struck late on Monday, citing US intelligence that led them to conclude Russia was to blame.

Moscow says the convoy was not hit from the air and has implied rebels were to blame, saying only rescue workers affiliated to the opposition knew what had happened. Russia's foreign ministry told reporters at the United Nations the US administration "has no facts" to support its assertions.

Aleppo, Syria's biggest city before the war, has been a focal point of the war this year as Assad and his allies have sought to encircle the insurgent-held east and cut opposition supply lines to Turkey.

Having blockaded eastern Aleppo, the government and its allies aim to clear insurgents from areas to the south and west, to take back territory including the main Damascus-Aleppo highway. Shi'ite militia from Iraq, Lebanon and Iran play a big role fighting on the government's side.

But rebel groups still have a strong presence in the area which abuts the insurgent stronghold of Idlib province. The powerful group formerly known as the Nusra Front has played a big role in fighting against the government.

Long Al-Qaeda's Syrian wing, Nusra has changed its name and disavowed Al-Qaeda, but is still characterised by both the West and Moscow as a terrorist group excluded from the ceasefire. Other rebels say Russia and the Syrian government exploit this to justify broader attacks.