The number of Syrians in need of humanitarian aid has shot up by 500,000 to 1.5 million in less than three months, the UN said on Friday.
"The humanitarian situation in Syria continues to deteriorate," the Office for the Coordination of Humanitarian Affairs (OCHA) said in an update.
"It is now estimated that up to 1.5 million people need humanitarian assistance."
The figure is up from the one million people experts said were in need at the end of March this year.
In Idlib, the Syrian Arab Red Crescent reported that 350,000 people are now in need, OCHA said, an increase of 150,000 since March. A further 250,000 people require help in Homs governate.
On June 5 the UN agreed with the Syrian authorities to establish four humanitarian sites to allow the delivery of essential aid.
A reconnaissance mission to Idlib, Deraa, Homs and Deir Ezzor has now been completed, OCHA spokesman Jens Laerke told reporters in Geneva, and hubs may initially be established in the latter two.
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But the deployment of staff to these field locations is on hold "given the deteriorating security situation" and even local agencies such as the Syrian Arab Red Crescent are having difficulty reaching those in need.
"People are trapped in Homs and unable to leave," said Laerke, who said that both the International Committee of the Red Cross (ICRC) and the Syrian Arab Red Crescent were unable to enter the old city of Homs on Thursday after two failed attempts.
"The overall problem is a lack of security ... guarantees could be provided by the Syrian government" he said.
Amid new clashes in Homs on Friday, hundreds of people are believed to be trapped in the historic heart of the city, unable to flee or find shelter, the ICRC said.
An estimated 168 people died in one of the bloodiest days of the 15-month uprising as Syrian troops clashed with rebels in the city's besieged Khalidiyeh neighbourhood, according to the Syrian Observatory for Human Rights.
In the last 15 months of fighting the death toll has risen to 15,026, the majority of whom were civilians, the British-based monitor said.
Meanwhile, the number of refugees fleeing the conflict to Jordan, Iraq, Lebanon and Turkey has reached more than 86,000, according to the UN's refugee agency, UNHCR.
Because of the continuing exodus of refugees the UNHCR has decided to maintain its regional response plan until the end of the year.
In Geneva, UN-Arab League envoy Kofi Annan will hold a press conference with the head of the UN mission in Syria Robert Mood on Friday at 1400 GMT.