On the train that travels from Dushanbe, the Tajik capital, to the Russian Caspian port of Astrakhan, officials leave no bag unchecked in their quest to uncover illegal narcotics.

After a three-month break, the train has only just resumed service. As it winds through central Asia - from Tajikistan, through Uzbekistan, Turkmenistan and Kazakhstan - it at times follows the ancient Silk Road, through the fabled cities of Bukhara and Samarkand.

The goods being transported, however, are no longer silks and spices. Locally the Dushanbe-Astrakhan service is known as the "drug train". It is one of the main conduits for Afghan heroin, starting in Tajikistan and reaching into Russia and ultimately western Europe.

About 700 passengers are making the three-day journey to Astrakhan on an unusually warm December day. Most appear to be young Tajik men, heading to Russia in search of employment or for small-time trading.

They sit sullenly as guards at the border crossing into Uzbekistan dig through their belongings and ask the purpose of their travel.

No narcotics are discovered this time. This does not mean, however, that they will not be found by Turkmen, Kazakh or Russian authorities further down the line or in the weeks to come.

Around 120kg was discovered under the floorboards of a carriage in Astrakhan last year.

The drugs are also smuggled in small amounts by couriers. More often, the heroin is carried in their bodies as well, either swallowed in small packets or inserted in various orifices. For the sum of $200 (£138), these "mom and pop smugglers", as one western diplomat described them, are willing to risk jail - or death if the packets burst.

"We take away for examination those people who look suspicious or under stress. Carrying heroin in your stomach is very difficult," says an Uzbek border guard.

There are no precise figures for the amount of narcotics passing through central Asia to the west. But officials from the United Nations Office for Drug Control and Crime Prevention, estimate that 100-120 tonnes of heroin pass through Tajikistan each year.

This would be enough potentially to meet all the heroin needs of Europe and the US, although the Tajik and some Russian authorities dispute the numbers.

If the figure is correct, this would make Tajikistan - a poor former Soviet republic bordering Afghanistan which until recently was ripped apart by civil war - the single largest transit point for Afghan heroin, surpassing Iran and Pakistan.

"It is very important to understand the magnitude of all this," says Roberto Arbitrio, the UN's programme co-ordinator in the anti-drug campaign. "

Tajikistan has emerged as an important transit point thanks to its proximity to heroin storage centres in northern Afghanistan, as well as the relative ease which smugglers can cross the 1,500km border.

Some narcotics do slip into Afghanistan's other central Asian neighbours, Turkmenistan and Uzbekistan, but authorities say most drugs seized in these two countries originated in Tajikistan.

From Tajikistan the heroin snakes its way into the neighbouring former Soviet states, travelling on passenger trains such as that between Dushanbe and Astrakhan, or by aircraft, car, cargo train, camel or donkey.

As a result, central Asia, a region which practically saw no drug use during the Soviet era and only recorded its first narcotics seizure in 1996, is now awash with drugs.

In each country the narcotics leave a wake of crime and corruption. Sometimes the trail leads all the way to the top, as is evident from the murder of Tajik deputy interior minister Habib Saginov inlast year, the victim of a heroin deal gone bad.

Rates of addiction and Aids cases, the result of intravenous drug use, have also begun to rise sharply. Although the number of registered addicts in each country is only a few thousand, authorities admit the real figures could be over 100,000.

Government officials say they recognise the depth of the problem, and claim it is within their means to deal with it. What is needed most is international aid.

"The most important thing is that the leadership of our country has announced to the entire world that we have a problem. We say this openly, we have nothing to hide," says Major Avaz Yuldashev, spokesman for the Tajik state drug control agency.

Anti-drug agencies across the region point to dramatic successes in recent months. Most prominent of these is the Russian federal border service, which patrols Tajikistan's frontier with Afghanistan.

Last year the Russian border guards seized more than 2.3 tonnes of heroin, up from 800kg the previous year, accounting for more than half Tajikistan's heroin haul.

Increased seizures could also in part be due to larger quantities of heroin being trafficked. This possibility is bolstered by the fact that large stockpiles in northern Afghanistan permitted heroin exports to continue uninterrupted in spite of a ban on poppy cultivation by the Taliban and the US bombing campaign.

And there is little indication that the numbers will decrease in the short term. One kilogram of heroin which costs $5,000 in Tajikistan can fetch $300,000 in Europe. In a region of poverty wages and high unemployment this is a mark-up few canresist.