UNITED NATIONS, Aug 24 (Reuters) - The United Nations will launch an appeal, expected to be for $15-20 million, early next week for aid to deal with the aftermath of flooding in North Korea, a senior U.N. official said on Friday.

Some of the worst flooding in years hit the impoverished communist country this month, ravaging farmland, destroying tens of thousands of homes and buildings and damaging roads, North Korea and aid agencies say.

The official death toll remains at 221, with about 400,000 people affected, U.N. Deputy Emergency Relief Coordinator Margareta Wahlstrom told a news conference.

"The waters are now receding, and ... many people in public buildings are now waiting for their homes to either be reachable ... or if they've been destroyed, to get temporary shelter," she said.

Wahlstrom said no exact figure for the appeal had yet been decided but it was expected to be somewhere between $15 million and $20 million. It would focus on immediate emergency needs such as food, medical supplies, water and sanitation.

"Anything else that goes beyond that will be dealt with later on in a different format," she said.

A South Korean minister said Seoul would send building materials and equipment worth nearly $40 million to the North. South Korea has been shipping in emergency aid this week.

An appeal this week by the International Federation of Red Cross and Red Crescent Societies for $5.5 million was "slowly getting a response", Wahlstrom said.

The week of torrential rain had landed some 33 inches (850 mm) on North Korea, at least half its normal annual rainfall, she said.

Although North Korea is one of the world's most closed societies, U.N. agencies had had no problems from authorities in visiting affected areas, Wahlstrom said.

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