Syrian government and rebel forces battled for control of high ground on the Aleppo outskirts on Saturday as warplanes bombed the city's opposition-held east relentlessly in a Russian-backed offensive that has left Washington's Syria policy in tatters.
In their first major ground advance of the offensive, the army and its militia allies seized control of the Handarat Palestinian refugee camp, a few kilometers north of Aleppo, only for rebels to counterattack a few hours later.
"The fighters are waging ferocious battles because it is a battle of existence," a senior rebel official told Reuters.
Rebels said they had recovered some or all of Handarat. The Syrian Observatory for Human Rights said the battle was ongoing. There was no immediate comment from the Syrian military.
The assault on Aleppo, where more than 250,000 civilians are trapped in a besieged opposition sector, could be the biggest battle yet in a civil war that has killed hundreds of thousands of people and driven 11 million from their homes.
Residents say air strikes on eastern Aleppo since the offensive was announced on Thursday have been more intense than ever, using more powerful bombs. Scores of people have been killed in the last two days.
Two weeks after Moscow and Washington announced a ceasefire, President Bashar al-Assad and his Russian and Iranian allies appear to have launched a campaign for a decisive battlefield victory that has buried any hope for diplomacy.
US Secretary of State John Kerry, who hammered out the truce over the course of months of intensive diplomacy, was left pleading in vain this week with Russia to halt air strikes.
Rebel officials said air strikes on Saturday hit at least four areas of the opposition-held east, and they believe the strikes are mostly being carried out by Russian warplanes. Video of the blast sites shows huge craters several meters wide and deep.
"There are planes in the sky now," Ammar al Selmo, the head of the Civil Defence rescue service in the opposition-held east, told Reuters from Aleppo on Saturday morning.
The group draws on ambulance workers and volunteers who dig survivors and the dead out of the rubble, often with their bare hands. It says several of its own centers have been destroyed in the latest bombing. "Our teams are responding but are not enough to cover this amount of catastrophe," Selmo said.
The Syrian Observatory for Human Rights said at least 45 people, among them 10 children, were killed in eastern Aleppo on Saturday. Selmo put the two-day death toll at more than 200.
The army says it is targeting only militants.
The war has ground on for nearly six years, with all diplomatic efforts collapsing in failure. Half of Syria's population has been made homeless, world powers and regional states have been drawn in, and Islamic State - the enemy of every other party to the conflict - has seized swathes of Syria and neighboring Iraq.
For most of that time, world powers seemed to accept that neither Assad nor his opponents were likely to be capable of decisive victory on the battlefield.
But Russia's apparent decision to abandon the peace process this week could reflect a change in that calculus and a view that victory is in reach, at least in the western cities where the overwhelming majority of Syrians live.
Assad's fortunes improved a year ago when Russia joined the war on his side. Since then, Washington has worked hard to negotiate peace with Moscow, producing two ceasefires. But both proved short-lived, with Assad, possibly scenting chances for more battlefield success, showing no sign of compromise.
Moscow says Washington failed to live up to its side of the latest deal by separating mainstream insurgents from hardened jihadists.
Outside Aleppo, anti-Assad fighters have been driven mostly into rural areas. Nevertheless, they remain a potent fighting force, which they demonstrated with an advance of their own on Saturday.
The Syrian Observatory for Human Rights monitoring group said rebels, including the jihadist Jund al-Aqsa group, had seized two villages in northern Hama province, an area that is strategically important and close to the coastal heartland of Assad's Alawite minority sect.
A Syrian military source said the army was "fighting fierce battles" around the two villages, Maan and al-Kabariya.
A rebel commander told Reuters he expected fighters would receive more weapons from sponsoring countries to counter the government's latest advance, although there was no sign they would obtain advanced arms such as anti-aircraft missiles they have long sought.
"There are indications and promises" of more weapons, though he expected only "a slight increase", said Colonel Fares al-Bayoush, head of the Northern Division rebel group. He expected more "heavy weapons, such as rocket launchers and artillery".
Damascus and its allies including Shi'ite militia from Iran, Iraq and Lebanon have encircled rebel-held areas of Aleppo gradually this year, achieving their long-held objective of fully besieging the area this summer with Russian air support.
A pro-government Iraqi militia commander in the Aleppo area told Reuters the aim was to capture all of Aleppo within a week.
A Western diplomat said on Friday the only way for the government to take the area quickly would be to totally destroy it in "such a monstrous atrocity that it would resonate for generations".
UN Secretary-General Ban Ki-moon condemned the "chilling military escalation" in Aleppo, his spokesman said on Saturday.
The United Nations Security Council is due to meet at 11 a.m. (1500 GMT) on Sunday to discuss the recent escalation of fighting in Aleppo, said diplomats.
The meeting, which will be public, was requested by the United States, Britain and France.
In a meeting on Saturday in Boston, Kerry and his counterparts from the EU, Britain, Germany, Italy and France called on Russia to "take extraordinary steps to restore the credibility of our efforts, including by halting the indiscriminate bombing by the Syrian regime of its own people, which has continually and egregiously undermined efforts to end this war."
UNICEF, the UN children's charity, said a pumping station providing water for rebel-held eastern Aleppo was destroyed by bombing, and the rebels had responded by shutting down a station supplying the rest of the city, leaving two million people without access to clean water.
Tarik Jasarevic, spokesman for the World Health Organisation, said on Saturday the water system was working "in around 80 percent of the city - both sides".
A Syrian military source told Reuters its operation was continuing according to plan, but declined to give further details. The source said on Friday the operation could go on for some time.
Asked about the weapons being used, the source said the army was using precise weapons "suitable for the nature of the targets being struck, according to the type of fortifications", such as tunnels and bunkers, and "specifically command centers".
In New York, Syrian Foreign Minister Walid al-Moalem told the UN General Assembly the Syrian government's belief in victory is even greater now that the Syrian army "is making great strides in its war against terrorism".
The EU foreign policy chief Federica Mogherini and Humanitarian Aid Commissioner Christos Stylianides said the Aleppo situation had "seen a dramatic and rapid deterioration".
"The human suffering being caused is an affront to the whole world. It risks to take us ever further from a negotiated settlement of the conflict, which remains the only way of bringing it to an end," they said in a joint statement.
