Vienna: The proliferation of new narcotics developed to circumvent existing drug laws poses a serious health problem, the United Nations drugs control body said on Tuesday.

The International Narcotics Control Board (INCB) tasked by the United Nations to oversee the existing legal vacuum with regards to the new drugs makes it easy to market these substances, often over the Internet.

"They can be natural materials or synthetic substances, often deliberately chemically engineered to circumvent existing international and domestic drug control measures," the INCB said in its latest report.

The problem is particularly serious in the United States where the phenomenon started around 10 years ago, and is quickly spreading to the rest of the world.

The number of new substances has doubled since 2009 with 388 new ones added to the list in October last year, the UN body said.

China is "one of the main sources of supply of new psychoactive substances", said the report.

The INCB also criticised the legalisation of cannabis in certain US states and in Uruguay, saying it went against international drug laws.

Increased levels of THC, the active ingredient in cannabis, is of particular concern, the INCB report said.

There has been a 37 per cent increase in the potency of THC in the drugs seized in the United States for example and a 75 per cent increase in cannabis coming from outside the country.