By Martin Plaut BBC correspondent in Lusaka, Zambia

A row over genetically modified food is threatening to derail efforts to help the 12 million people across southern Africa who are facing a critical food shortage.

The United Nations' attempt to bring relief aid to the region appears to have hit an unexpected problem: the fact that its largest donor, the United States, is providing genetically modified grain.

And a number of governments in southern Africa are less than enthusiastic about the possibility of their own crops being contaminated by the American aid.

Mozambique is refusing to allow its ports to be used for trans-shipment to Malawi and Zimbabwe, two of the worst-hit countries in the region.

Change of plans

For Richard Ragan, the UN World Food Programme representative in Zambia, this could mean re-thinking the whole transport strategy.

"If GMO maize can't transit Mozambique, and it's likely it would have to come in from Dar es Salaam, so that means that we have to begin to consider different options with respect to both rail and trucking opportunities in the region," he said.

One possibility would be to grind the maize before it arrives in southern Africa.

This could not only be costly, but also logistically difficult.

And aid agencies worry that the United States might not accept conditions being imposed on it donations.