The death toll from two days of bombing by Syria's government of a rebel-held area has risen to 250, reports say.
It is the worst violence in the Eastern Ghouta area near Damascus since 2013, according to activists. More than 50 children are among the dead, they say.
The UN has warned that the situation is "spiralling out of control".
Meanwhile the Damascus government has sent forces to confront Turkish troops who have crossed the border to push back the Kurds in northern Syria.
Turkey fired shells near the advancing columns, which, it claims, forced the pro-government fighters into retreat.
The Syrian military has not commented on the reports from the Eastern Ghouta but says it carried out "precision strikes" on areas from which the shells were launched.
A UN spokesperson said at least six hospitals had been hit in the area on Monday and Tuesday.
What's happening in the Eastern Ghouta?
Pro-government forces - backed by Russia - intensified their efforts to retake the last major rebel stronghold on Sunday night.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said at least 250 people had been killed in air strikes and artillery fire since then.
It said it was the highest 48-hour death toll since a 2013 chemical attack on the besieged enclave. About 1,200 people were injured.
Activists say at least 10 towns and villages across the Eastern Ghouta came under renewed bombardment on Tuesday.
The UN called for a ceasefire to allow humanitarian aid to be delivered and the wounded to be evacuated.
New players are shifting the war
By Jeremy Bowen, BBC Middle East editor
After seven years, Syria's war is not ending but it is changing.
President Assad now looks like he is trying to roll up this final major enclave around Damascus - the Eastern Ghouta. This would secure his victory around the capital, and would be a very big moment.
But Syria remains linked into a web of war and power politics, which guarantees more conflict.
Up in the north, there are a whole host of big powers completing for influence: Russia, Iran, Turkey and the United States. British Special Forces are there, too.
Iran is seen as a particularly big threat by the Americans, the Israelis, who are also getting involved increasingly, and the Saudis, who have been big players in the war as well.
While the cast of characters is changing, the bloodshed continues and it is certainly not over.
How bad is the situation in the enclave?
A local doctor told the Union of Medical Care and Relief Organisations, which supports medical facilities in the Eastern Ghouta, that it was "catastrophic".
"People have nowhere to turn," he said. "They are trying to survive but their hunger from the siege has weakened them significantly."
The UN's co-ordinator in Syria, Panos Moumtzis, said he was "appalled" by reports that hospitals had been deliberately targeted, warning that such attacks might amount to war crimes.
Five hospitals in Marj, Saqba and Douma were left inoperable or partially functioning after reported government strikes on Monday, while on Tuesday a hospital in Zamalka was hit, according to Mr Moumtzis.
The Syrian American Medical Society said a hospital in Arbin was also put out of service on Tuesday. The Syrian Observatory said the facility had been targeted by Russian warplanes.
The government has allowed one humanitarian convoy into the Eastern Ghouta since late November, and there are severe shortages of food.
A bundle of bread now costs close to 22 times the national average and 12% of children under five years old are said to be acutely malnourished.
The Eastern Ghouta is dominated by the Islamist faction Jaysh al-Islam. But Hayat Tahrir al-Sham, a jihadist alliance led by al-Qaeda's former affiliate in Syria, also operates there.
The region has been designated a "de-escalation zone" by Russia and Iran, the government's main allies, along with Turkey, which backs the rebels, but hostilities intensified in mid-November.
What else is going on in Syria?
On Tuesday, Syrian pro-government forces entered the Kurdish enclave of Afrin, just south of the Turkish border.
Turkey is trying to oust the Kurdish militia, which have semi-autonomous rule of the area and which have called on the Syrian military for help.
Syria has denounced the Turkish offensive as a "blatant attack" on its sovereignty, while Turkey has insisted it will not back down.
Syrian government forces, supported by Russian air strikes and Iran-backed militias, are also carrying out offensives on the north-western province of Idlib.
The UN says more than 300,000 people have been displaced by the fighting in Idlib since December.
It gave no details, but it is believed to be a reference to an incident in the eastern province of Deir al-Zour on 7 February, when the US military said it had killed an estimated 100 pro-Syrian government fighters in response to an attack on an allied, Kurdish-led militia force battling Islamic State militants in the area.