The United Nations said Tuesday that recent monsoon flooding in Sri Lanka had hit hundreds of thousands of victims who were forced from their homes just weeks ago.

"The impact on people of this second wave of floods is even greater than the first in large part as peoples' capacity to cope was already diminished," UN Colombo chief Neil Buhne said in a statement.

"The UN will revise its flash appeal of $51 million in emergency funding at the end of this month in accordance with needs of both flood events."

More than 300,000 people in the north, east and centre are currently displaced, with thousands of houses destroyed and over a million people directly affected by the floods.

The government's Disaster Management Centre on Tuesday reported at least 19 flood-related deaths. Last month's flooding claimed at least 43 lives.

Oxfam said that the destruction of rice fields had left Sri Lanka with long-term food security issues in addition to the immediate humanitarian needs.

"The floods are likely to destroy more than 90 per cent of the (rice) paddy cultivation in the affected districts," Oxfam's manager in Sri Lanka, Thusitha Siriwardana, said.