By Shapi Shacinda

LUSAKA, March 26 (Reuters) - Zambia has put the military on high alert to help aid agencies tackle the worst floods in four decades, which have displaced over 20,000 families and submerged crops ahead of the growing season, officials said on Thursday.

The air force is on standby to airlift people to safety from waters that are still rising as recent heavy rains swell the Zambezi and its tributaries. Namibia, also hit by floods, has declared a state of emergency in affected areas.

Loyce Saili, spokesperson for the Road Development Agency, told state television the recently rebuilt road linking Zambia to Zimbabwe and the rest of southern Africa had been washed away, slowing the landlocked country's copper exports.

"We are in a chaotic situation and the president has placed the military on high alert to help (aid and) government agencies dealing with the floods to cope with this problem," a senior security official told Reuters.

Water levels in some districts are at their highest since 1969 and officials say there may be an urgent need for food in some districts.

Gaston Sichilima, the junior minister for disaster relief, said the floods would cause further damage to crops and houses when they reached the lower reaches of the Zambezi.

"When flying over the (affected) areas in western province, we saw . . . rice and maize fields submerged due to rainfall patterns we last experienced in 1969," said Sichilima.

Water levels were expected to rise further when other major rivers discharged more water into the Zambezi.

In January, rains in Malawi and Zambia caused flooding in Mozambique that killed 45 people and left 285,000 homeless, the worst floods to hit the country since 2000-2001 when 700 people died and half a million lost their homes.

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