The looming exit of foreign troops from Afghanistan will force the United Nations to take a larger role fighting poppy cultivation in Afghanistan and trafficking into neighboring countries, the top U.N. anti-narcotics official said on Monday.

But Yuri Fedotov, the head of the UN Office of Drugs and Crime also highlighted rare, and increasing cooperation on anti-narcotics work between Afghanistan and its neighbours Iran and Pakistan, who have troubled relationships marked more often by distrust than cooperation.

Unprecedented regional steps that have taken place so far include sharing radio frequencies which were once kept secret.

The level of mistrust was so high and the countries have a long history of mutual suspicion, Fedotov told Reuters in an interview in Kabul to bolster trilateral drug-control efforts.

When we started it was difficult to talk about intelligence sharing, joint operations. It took a long time for them to realise that is in their common interest, he said of the triangular initiative, which began in 2007.

At the latest meeting of top officials from all three countries they agreed to increase joint intelligence, raids, and border patrols in areas where opium flows out of Afghanistan.

But with foreign combat forces, and much of their cash and air power, expected to be gone from the country by the end of 2014, the Afghan government will likely need more help fighting poppy cultivation, which rose over the last year.

A former minister of counter-narcotics told Reuters last week that production will likely rocket when Afghan security forces are in sole charge of their country. Fedotov said the United Nations might be able to provide some extra help.

We will see it as an additional incentive for us to do more and to deliver more, Fedotov told Reuters when asked about the impact of a security transition agreed by both Afghan President Hamid Karzai and the nations whose troops are fighting there.

The UN will have more responsibility in helping the government of Afghanistan to continue to meet the challenge of drugs, he added.

Land under poppy cultivation climbed 7 per cent from 2010 and the crop returned to three provinces in the north and east that had been declared poppy-free, according to a joint report by the U.N. drugs agency and Afghanistan's counter-narcotics ministry released in October.

The increase came even though crop eradication was 65 percent higher than a year ago. The report mostly blamed a plant disease the previous year for the rise in the poppy economy as farmers sought to capitalise on soaring prices.

There is also a large foreign-funded push to wean farmers off poppy, a hardy crop that needs relatively little water, by offering incentives -- such as subsidized seed and fertilizer -- to grow legal crops.

But continued insecurity in many poppy producing provinces and uncertainty over whether the Taliban could return to power after foreign troops leave only adds to problems convincing farmers to change their crops to legal crops such as wheat.

Of course, the income could not be competitive or compatible with the income that you have from this illicit business, but instead they will get more stability and a more predictable future, Fedotov said about why farmers should switch crops.

Corruption and weak commitment by the Afghan government continue to be major obstacles to reducing poppy cultivation in Afghanistan, the world's leading producer of opium, he said.

It is ... another consequence of illicit business, which is cultivating poppy and producing opium; corruption is a part of that, he said.

What is required is more political commitment.

(Reporting By Christine Kearney, editing By Emma Graham-Harrison)