GENEVA: The UN food agency said Tuesday it will dramatically expand its aid operation in Syria to reach one million more people amid growing alarm over the humanitarian situation in the war-torn country.

"In February, we hope to scale up our operations and reach 1.75 million, then two million in March, and 2.5 million vulnerable Syrians in April," said Elisabeth Byrs, a spokeswoman for the World Food Programme.

Since September, the WFP has been feeding some 1.5 million vulnerable Syrians, sending around 400 truckloads of food each month.

Access to opposition-controlled areas has been particularly difficult, as it involves crossing battle frontlines, while the Syrian government has maintained restrictions on international aid group operations.

Byrs said the WFP had been able to send supplies to between 40 and 50 percent of opposition-controlled areas, and was also reaching government areas, although she did not elaborate.

"Aid is based on need, not political labels," she said, while noting: "But there are of course danger zones which can't be reached because of fighting."

Byrs said the Syrian government had given WFP a green light to import 2.5 million litres (660,000 gallons) of fuel per month to power its aid trucks.

"The first consignment of 39,000 litres arrived in Syria from Lebanon two weeks ago," she said.

Jens Laerke, a spokesman for the UN Office for the Coordination of Humanitarian Affairs, said the situation in Syria had deteriorated sharply.

"We've called the situation critical before. We're at the point where we're calling it catastrophic," he told reporters in Geneva.

"If the violence continues unabated, in the short term we could see considerably more than the current four million in need of urgent assistance, and more than two million internally displaced in Syria," Laerke said.

"In the face of this human tragedy, organisations are struggling to reach more people, in more places, with more aid, but lack of access is still a major obstacle."

Speaking by telephone from the Syrian capital Damascus, World Health Organisation representative Elizabeth Hoff warned that cases of waterborne diseases could spiral.

"The biggest concern is the water and sanitation system," Hoff told reporters, pointing to a shortfall in chlorine needed to purify drinking supplies.

The Syrian government has lost control of several reservoirs and can no longer ensure their proper functioning, she noted.

On Monday, the United Nations announced it was delivering purification chemicals to help supply clean water to some 10 million Syrians, or around half the population.

Hoff also highlighted concerns about an increase in the number of injured civilians, notably with severe burns.

The United Nations estimates that more than 60,000 people have been killed since the revolt against President Bashar al-Assad's regime began in March 2011, and more than 730,000 have fled to neighbouring countries.