The divided Syrian city of Aleppo has seen a morning of air attacks, with reports of barrel bombs hitting a hospital already bombed this week.
The Russian and Syrian air forces have been attacking the rebel-held east of the city, causing international outrage at the mounting civilian death toll.
Government forces began a ground offensive against the rebels more than a week ago as a truce collapsed.
The US says Russia is driving moderate rebels into the arms of jihadists.
Once Syria’s commercial and industrial hub, Aleppo has been divided roughly in two since 2012.
The UN says at least 400 civilians, including many children, have been killed in the city this week as a result of Russian and Syrian government attacks.
Rebel shelling has also claimed civilian lives in the east, reports say.
A spokesman for the Syrian American Medical Society, a non-governmental organisation, told AFP news agency two barrel bombs – improvised devices dropped from helicopters – had hit a hospital it supports.
“There were reports of a cluster bomb as well,” Adham Sahloul said.
The M10 hospital was said to be out of service after it was bombarded on Wednesday but Mr Sahloul said a small group of patients and staff had been inside when the bombing began and remained trapped there.
Hospital administrator Mohammad Abu Rajab made an urgent appeal for help on Saturday morning from inside M10, AFP adds.
“The hospital is being destroyed! SOS, everyone!” he was quoted as saying.
Jets also bombed rebel-held areas north of the city, according to activists and rebel sources, while on the ground fighting raged near Aleppo’s Old City.
The army said it had gained ground but the rebels deny this.
“They are shelling the old city heavily after another failed attempt to gain ground,” Abu Hamam, from the Failaq al-Sham group, was quoted as saying by Reuters news agency.
“They have lost several fighters and we are steadfast.”
The Syrian Observatory for Human Rights, a UK-based opposition monitoring group, also reported government barrel bomb and jet attacks on the Ghouta area outside Damascus on Saturday.
In another development, it said government forces were battling fighters from so-called Islamic State in Homs region.
War of words
Washington and Moscow have continued to spar over Syria, with the US dismissing Russian accusations that it was protecting a jihadist group in its bid to oust Syrian President Bashar al-Assad.
Russian Foreign Minister Sergei Lavrov said the US had broken its promise to separate the powerful Jabhat Fateh al-Sham (formerly known as al-Nusra Front) and other extremist groups from more moderate rebels.
State department spokesman Mark Toner said the Russian allegations were “absurd”.
He told reporters the US had not targeted al-Nusra for months because they had become “intermingled” with other groups and civilians.
The US says it may end co-operation with Russia over its action in Syria.
Mr Lavrov made his claims of US broken promises during an interview with Stephen Sackur on BBC World News TV on the first anniversary of the beginning of the Russian air campaign in Syria.
“They [the US] pledged solemnly to take as a priority an obligation to separate the opposition from Nusra,” he said.
“They still, in spite of many repeated promises and commitments… are not able or not willing to do this.”
At least 250,000 people have been killed in the conflict since it began in March 2011 with the Observatory estimating the true number to be around 430,000.
More than 4.8 million people have fled abroad, and an estimated 6.5 million others have been displaced within the country, the UN says.