The Syrian Arab Red Crescent estimates 2.5 million people have been displaced within Syria, doubling the previous figure used by aid agencies.
UN refugee agency spokeswoman Melissa Fleming said that the group's estimate was "very conservative" and warned that it might be far higher in reality.
People on the run or in hiding were difficult to count and help, she added.
Meanwhile, Syria's new opposition coalition says it wants to be recognised as the country's government.
This would enable it to buy weapons to assist its attempts to overthrow President Bashar al-Assad.
Opposition and human rights activists estimate that more than 36,000 people have been killed since the uprising against President Assad began in March 2011.
More than 408,000 Syrians have fled to neighbouring countries, and more are fleeing every day, according to the UN.
However, far more people have left their homes but stayed inside Syria, and humanitarian agencies have struggled to help them.
In Geneva on Tuesday, the chief spokeswoman of the UN High Commissioner for Refugees (UNHCR) said the Syrian Red Crescent had doubled its estimate of the number of internally displaced people (IDPs).
"The figure they are using is 2.5 million. If anything, they believe it could be more, this is a very conservative estimate," Melissa Fleming said.
"So people are moving, really on the run, hiding. They are difficult to count and access."
She added: "It's easier to count if people are living in refugee camps."
Many of the IDPs are located in schools and public buildings, which often lack adequate heating and sanitation facilities, according to the UN.
Most IDPs are hosted in local communities whose capacity to support them has been stretched, giving the shortages of water, food and medicines and dwindling income.
The UN believes up to four million people inside Syria will need humanitarian aid by early next year, up from 2.5 million.
Ms Fleming said recent deliveries of aid had been "very difficult".
A SARC warehouse in Aleppo was shelled and 13,000 blankets burned, Damascus operations were disrupted for two days and a lorry carrying 600 blankets was hijacked outside the capital. UNHCR staff have also been temporarily withdrawn from Hassakeh.
'Huge step forward'
The head of Syria's new opposition coalition, Mouaz Alkhatib, called for diplomatic support as Arab and European ministers met in Cairo.
"I request European states to grant political recognition to the coalition as the legitimate representative of the Syrian people and to give it financial support," he said in an interview with Reuters news agency. "When we get political recognition, this will allow the coalition to act as a government and hence acquire weapons and this will solve our problems."
French Foreign Minister Laurent Fabius appeared to provide backing.
"Our hope is that the different countries recognise the Syrian national coalition as the legitimate representative of the Syrian people... France's role is to make that hope possible," he told reporters in Cairo. He added: "The opposition has taken a huge step forward."
British Foreign Secretary William Hague said the new coalition could win official international recognition, but would need to show it had sufficient support inside Syria.
For now, he added, the European Union continued to operate an arms embargo and the focus remained on non-lethal support for the opposition, and humanitarian assistance for refugees.
"We want to see that they have support inside Syria. That is a very crucial consideration. If they do all these things, well then, yes, we will be able to recognise them as the legitimate representative of the Syrian people."
In Syria itself, fighting in and around the capital, Damascus, is reported to have killed at least 41 people, mostly civilians.
And Syrian government warplanes bombed a rebel-held town near the border with Turkey for a second day.
Plumes of smoke could be seen rising into the sky from the town of Ras al-Ain, in the north-eastern province of Hassakeh, and Turkish ambulances crossed the frontier to transport wounded Syrians to Turkish hospitals.
Turkey's Foreign Minister, Ahmet Davutoglu, said on Monday that it had lodged a formal protest with the Syrian government about the air strikes, which he said were endangering its security.
Iran, an ally of President Assad, has said it will host a meeting of the parties to the Syrian conflict in Tehran on Sunday.
The meeting will focus on promoting diplomacy and ending the violence in the country, Deputy Foreign Minister Hossein Amir Abdollahian told the Arabic-language al-Alam channel.