Aid groups need $150m (€120.04m) to provide urgent relief to 250,000 people displaced by separate offensives by the Syrian government outside Damascus and Turkish-led forces in the north, a UN Syria official said.
UN co-ordinator Ali al-Za'atari said some 80,000 people have fled the government's offensive in the eastern Ghouta region east of Damascus, where shelling and airstrikes have killed some 1,600 people in five weeks.
He said 50,000 were still living in shelters that have been stretched beyond capacity.
Around 26,000 people had returned to their towns after they were recaptured by the government, said Khaled Hboubati, the head of the Syrian Arab Red Crescent, at a joint press conference with Mr al-Za'atari in Damascus.
The International Red Cross (ICRC) said on Monday that children were emerging from eastern Ghouta with diarrhoea, lice and skin diseases.
It said many walked for miles on bare feet to reach the shelters.
The shelters were overcrowded, had no proper sanitation, and lacked toilets and showers, said the ICRC. It said it was working with the Red Crescent to improve them.
The Russian military operation in Syria said more than 120,000 residents had arrived on the government's side of the front lines around eastern Ghouta.
The Syrian Observatory for Human Rights monitoring group said 100,000 had crossed over. None of the figures could be independently verified.
Another 180,000 displaced Syrians are in need in the northern town of Tel Rifaat, after Turkish forces seized the Kurdish-controlled town of Afrin, said Mr al-Za'atari.
He told reporters in Damascus that "finance is coming, but is still beneath the required level".
A renewed push by the Syrian government and Russia to take the last rebel-held pocket in eastern Ghouta could drive tens of thousands more out of their homes and into shelters.
Russia gave the rebel Army of Islam faction 48 hours from Tuesday to agree to leave the town of Douma or face one last assault that could kill countless civilians trapped inside.
The government was amassing its forces around the town in preparation.
The Army of Islam is the last faction holding out against the government in eastern Ghouta.
If the fighters agree to leave, they will follow some 25,000 others - fighters and civilians - who have elected to board buses to rebel-held north-west Syria instead of reconcile with the government.
Many say they cannot serve in President Bashar Assad's conscription army. Others say they cannot trust the government's notorious security services.
The bus evacuations continued on Wednesday. Some 6,500 people arrived in Idlib from the formerly besieged Ghouta towns of Arbeen, Ein Terma, Zamalka, and Jobar.
Another convoy of buses was scheduled to leave eastern Ghouta later in the day, or early on Thursday.