VIENNA (REUTERS) - Uruguay's decision to legalise marijuana is in violation of an international convention on drug control, a Vienna-based body set up to monitor government compliance with such treaties said on Wednesday.

Uruguay became the first country to legalise the growing, sale and smoking of marijuana on Tuesday, in a pioneering experiment that will be closely watched by other nations debating drug liberalisation.

A government-sponsored Bill approved in the Senate provides for regulation of the cultivation, distribution and consumption of marijuana and is aimed at wresting the business from criminals in the small South American nation.

But the International Narcotics Control Board (INCB) said the legislation contravenes the 1961 Single Convention on Narcotic Drugs, to which it said Uruguay is a party. "Cannabis is controlled under the 1961 Convention, which requires States Parties to limit its use to medical and scientific purposes, due to its dependence-producing potential," INCB president Raymond Yans said in a statement.

He was surprised, the statement added, that Uruguay's legislature and government "knowingly decided to break the universally agreed and internationally endorsed legal provisions of the treaty".

The INCB describes itself as an independent, quasi-judicial body charged with promoting and monitoring compliance with the three international drug control conventions, including the 1961 Single Convention on Narcotic Drugs.

Uruguay's attempt to quell drug trafficking is being followed closely in Latin America, where the legalisation of some narcotics is being increasingly seen by regional leaders as a possible way to end the violence spawned by the cocaine trade.

Rich countries debating legalisation of pot are also watching the Bill, which philanthropist George Soros has supported as an "experiment" that could provide an alternative to the failed U.S.-led policies of the long "war on drugs".