Relief workers struggled Friday to set up a distribution network to get urgently needed food, medicine and water to millions of victims of the worst floods in Bangladesh history.

The government raised the flood-related death toll Thursday to 510, including those killed in boat capsizings, the collapse of houses and bites from snakes flushed from their lairs.Newspapers, however, reported more than 1,200 had died in the worst floods ever to devastate the impoverished south Asian nation of 110 million people,

It is generally accepted that all statistics are questionable because of the widespread dislocation of communications during the disaster.

Water levels in most of the north and central areas of the country were reported receding. And after seven days of limited operations because of runway flooding, the government reopened Dhaka's Zia International Airport to more than the small planes accorded access until now.

But officials said today water levels in northern and central areas of the country were dropping more slowly because of higher-than-normal tides that were stalling drainage into the Bay of Bengal, exacerbating conditions in the inundated south.

The Water Control Board said widely scattered rains did not significantly affect water levels in central and northern areas, but dumped up to 3 1/2 inches of water in catchment areas of the Meghna River - one of the country's three main waterways - which would contribute to flooding in the south.

"The general situation is improving in northern and central Bangladesh," said Sharif Ahmen, deputy director of the Water Control Board. "But in the south, the situation is deteriorating."

He said the Ganges and the Brahmaputra rivers had dropped only 6 to 8 inches in the 24-hour period ending this morning, compared to reductions as high as 10 inches during the previous three days.

Ahmed said the drainage of the rivers and their numerous tributaries was slowed by higher tides.