International criminal gangs are rapidly expanding into the burgeoning market for new types of legal highs, while law enforcement agencies lack the tools needed to keep up, the head of US overseas drug enforcement has warned.

Governments have struggled to keep up with the rapidly growing market for new psychoactive substances, as banning a new drug can require a complex legislative process and many of these drugs remain legal in some countries, said Brian Nichols, assistant secretary at the US Bureau of International Narcotics and Law Enforcement Affairs.

"These types of drugs are what transnational criminal networks are increasingly moving towards. Traditional drugs like marijuana are not as much in favour – they are bulky and hard to transport. Heroin and cocaine are very important but drug addiction is moving to the illicit use of pharmaceuticals and new substances like GBH," he told the Guardian.

"This is the growing threat. The use of traditional drugs is declining in the UK and the US, cocaine use is dropping, but prescription drug abuse is growing and new substance abuse is growing."

Websites offering new psychoactive substances, marketed as bath salts or plant foods, are proliferating, thanks in part to the failure of law enforcement agencies to keep up with the range of new chemicals. Dealers remain a step ahead of the law by slightly altering the formula for known molecules such as MDMA (ecstasy), ketamine or LSD to create new drugs. They can be far more dangerous than traditional drugs, because they have not been widely tested on the street and because the difference between a dose that supplies a high and one that results in fatality can be extremely small.

What was once a cottage industry has rapidly evolved, with labs and factories in China, Europe and the US manufacturing the chemicals on an industrial basis, churning out hundreds of tonnes of the compounds and selling them over the internet. It is this massive expansion of the trade that has attracted the attention of international drugs gangs, who use their expertise in trafficking traditional drugs such as heroin and cocaine to move into a new and lucrative market, said Nichols.

"There was a period of time in the US when you had new substances each week. Now you have by some counts well over 200 psychoactives [that] have been identified. It's my belief there are many more out there. We do not have people testing everything they come across," Nichols warned.

As with some other areas of international crime, such as wildlife trafficking, for which Nichols is also responsible, the rise of the internet has been a central factor.

"Cybercrime means people can order up crime online. It is a greater globalisation [of crime] than we have ever seen before," said Nichols.

But many of the buyers do not realise how dangerous the substances they are taking can be. "Some of these party drugs are an incredible high at the right dosage, but if you take [a fractional amount more] then you have an incredible toxin," Nichols said.

Nichols wants other countries to follow the lead of the US by bringing in legislation to fast-track the banning of new drugs. In the US, a process known as the scheduling of analogues allows drugs that are similar in effect or chemical make-up to existing illegal drugs to be banned without a lengthy process.

He also called for much more international co-operation in tracking and identifying new drugs and trying to prevent their distribution.

"One of the efforts we are pioneering in the UK and other partners in the G8 is encouraging the World Health Organisation to dedicate increased resources to identifying and scheduling of new psychoactive substances [and] create a more robust regime." He said there would also be an emphasis on demand reduction and treatment as well as preventing the sale and use of such drugs, and that help would be made available to countries lacking expertise in these areas.