International aid for 1.2 million Sri Lankans affected by monsoon flooding has fallen far short of the immediate humanitarian needs, United Nations figures showed Wednesday.

The UN said it received only $8.4 million as of Tuesday in response to an appeal for $51 million to meet urgent relief requirements in the island's north, centre and east.

Fresh flooding has submerged large areas of the country's rice-growing area, just as villages were trying to recover from devastating floods last month.

"Donors have provided $8.4 million of the $51 million requested by humanitarian agencies last month to respond to the flood crisis," the UN said, adding further money was expected.

Oxfam said it was supporting about 120,000 people, many of whom have been flooded out of their homes twice in the last six weeks.

"The international community needs to do more," Oxfam's country manager Thusitha Siriwardana said.

Sri Lanka's highways ministry said it would need at least $45 million to carry out urgent repairs to damaged highways.

Official figures showed there were at least 21 deaths related to the latest wave of flooding.

Water levels were receding with an improvement in the weather, though officials said 170,000 people are still in state-run welfare centres.

Last month's flooding claimed 43 lives.