By Adam Mynott BBC correspondent in Afghanistan

The world market is in danger of being flooded with heroin due to growing production of opium in Afghanistan, UN officials have warned.

The International Narcotics Control Board says that illegal opium poppy cultivation has spread in Afghanistan since last September, when the Taleban came under attack from the US-led coalition.

The Taleban had banned the growing of opium in areas under their control and had reduced the production of raw opium from 3,276 metric tonnes in 2000 to 185 tonnes last year.

The Taleban banned opium production

Opium poppies have a financial yield per hectare of more than 10 times that of other crops such as wheat, and there is clear evidence across the country that they are being planted again.

Some farmers in Nangarhar Province in eastern Afghanistan have sown a third of their land with poppy seed, and they threaten to cultivate more next year.

The Afghan Government hopes that with international financial assistance, it will be able to subsidise conventional crops and persuade farmers to try other forms of agricultural production.

Francis Mertons of the UN Office for Drug Control in Kabul says it is a problem that can be tackled.

"I believe that the international community could minimise the amount of production of opium in this country. It's a question of channelling humanitarian assistance combined with government action, relying primarily on the local authorities."