* One in three people deemed in need of help in Syria
* U.N. faces bureaucratic hurdles to deliver aid
* But more convoys crossing front lines, it says
GENEVA, May 7 (Reuters) - Bureaucratic hurdles still hamper delivery of aid in Syria where nearly one in three people need help, half of them children, the United Nations said on Tuesday.
Aid requirements have risen dramatically in the past year as the civil war has escalated, with some 6.8 million deemed in need now compared with 1 million in March 2012, the U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said.
Despite the lack of security, U.N. aid convoys are reaching people in hard-to-access areas such as Homs, but still face lengthy clearance procedures, it said.
"There has not been much movement on the bureaucratic obstacles, there is still a need for authorisations at several levels for each convoy, something we continue to work on with the Syrian government," OCHA spokesman Jens Laerke told a news briefing in Geneva.
He later told Reuters: "We need to give 72 hours advance notice for convoys. If there are smaller changes we have to restart the whole process. It's very, very slow."
The last convoy to cross the frontlines dividing Syrian government forces and rebels arrived in Ter Mallah and Al Ghan in Homs province on April 25, bringing food and other supplies for 24,000, the OCHA update said.
In all, 10 U.N.-led convoys delivered goods for 764,000 people in hot spots between January and April, it said. Five went to opposition-controlled areas and five to contested zones.
Rebels, mostly from the Sunni Muslim majority, hold chunks of southern, eastern and northern Syria, including about half of Aleppo, the country's biggest city.
But the government of President Bashar al-Assad still refuses to allow U.N. convoys to deliver aid to rebel-held areas by crossing from neighbouring countries such as Turkey.
Food rations from the U.N. World Food Programme reached 2.25 million people throughout Syria in April, short of its target of 2.5 million due to "access constraints" that led to the suspension of WFP operations for one week, OCHA said.
Syrians are on the move within the country on a massive scale, many uprooted multiple times to flee fighting, it said.
The estimated 4.25 million internally displaced include 1.25 million around Aleppo and more than 700,000 in Rural Damascus.
The United Nations estimated in mid-February that 70,000 people had been killed in the conflict that began in March 2011.
OCHA, referring to reports of mass killings in Mediterranean coastal areas of Baida and Banias last week, said that a first wave of civilians fled Banias for Tartous City on Saturday, but that some had been unable to cross government checkpoints.
Thousands more were expected to follow amid fears of further violence, it said.
More than 1.4 million Syrian refugees have registered or await processing in four neighbouring countries - Iraq, Jordan, Lebanon and Turkey - as well as North Africa led by Egypt. (Editing by Alison Williams)