By Alister Doyle, Environment Correspondent

CANCUN, Dec 2 (Reuters) - African nations can break dependence on food imports and produce enough to feed a growing population within a generation despite extra strains from climate change, a study said on Thursday.

Research into new crops resistant to heat, droughts or floods, better support for small-scale farmers and greater involvement by national leaders in setting policies in sectors from transport to education were needed, it said.

"Africa can feed itself. And it can make the transition from hungry importer to self-sufficiency in a single generation," said an international study led by Harvard University professor Calestous Juma.

About 70 percent of Africans are involved in agriculture but almost 250 million people, or a quarter of the population of the poorest continent, are undernourished. The number has risen by 100 million since 1990.

Juma, who is a professor of international development, told Reuters that food self-sufficiency would require big shifts in policies that have led to dependence on food aid handouts and imports in many nations.

"Climate change makes it more difficult," he said in a telephone interview of the study released to coincide with a meeting of several African leaders in Tanzania on Thursday, as well as U.N. talks on slowing climate change in Cancun, Mexico.

The U.N. panel of climate scientists says that up to 220 million people in Africa could face extra disruptions to water supplies by 2020. It says the continent faces more heatwaves, floods, mudslides, desertification and droughts.

Juma said the study, "The New Harvest, Agricultural Innovation in Africa," called for more involvement by national leaders in solving problems in sectors such as water, energy, transport, communications and education.

He said that the army, for instance, might refuse if the agriculture minister asked them to build a new road vital to distribute food. "But if the president asks they will do it. The president is the commander in chief," he said.

Research, including genetic modification, could help by developing new crops, perhaps by exploiting traits in indigenous varieties, he said.

"New technologies, especially biotechnology, provide African countries with additional tools for improving the welfare of farmers," said Burkina Faso President Blaise Compaore in a statement about the study.

And any methods developed by Africa could help other parts of the world. "It will pave the way for improved collaboration between Africa and South America," said Costa Rican President Laura Chincilla