The situation in the Syrian rebel enclave of the Eastern Ghouta is "beyond imagination", the UN's co-ordinator in Syria says, following days of bombing by Syria's government.
Panos Moumtzis told the BBC the bombardment near the capital Damascus, said to have killed at least 250 people, had caused "extreme suffering".
The Syrian military says it is trying to liberate the area from terrorists.
Meanwhile Syrian troops have been sent to confront Turkey in the north.
The Turks have crossed the border to push back the Kurds in northern Syria.
Turkey fired shells near the advancing columns, which, it says, forced the pro-government fighters into retreat.
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.
One resident, Firas Abdullah, told the BBC there was nowhere to hide: "We can hear the shout and crying of women and children through their windows of their homes.
"And the missiles and the mortars are dropping on us like rain. There is nowhere to hide from this nightmare and it isn't over."
More than 50 children were among the dead, according to activists. About 1,200 people were injured.
The UN has called for a ceasefire to allow humanitarian aid to be delivered and the wounded to be evacuated.
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.