US attorney general Eric Holder is preparing to announce his response to the decision of voters in Colorado and Washington state to legalise cannabis, as the Obama administration comes under mounting pressure within the US and around the world to stop the creeping legalisation of the drug.

Holder has let it be known that his review into last November's vote in the two states to legalise marijuana is coming to an end, and that he will make an announcement of his official response soon. Some observers expect that to be as early as Wednesday, when the country's top law officer faces questioning from the Senate judiciary committee in Washington DC.

As Holder completes his legal review of developments in Colorado and Washington state, eight former heads of the Drug Enforcement Administration (DEA), the federal body tasked with clamping down on illegal drug use, urged Obama to act quickly to bring the states into line. Cannabis use remains banned under federal law, which pits the Obama administration against the two states whose voters opted to allow the drug for recreational use.

In a letter, the six former DEA chiefs, who worked under a succession of both Democratic and Republican presidents, urged the Department of Justice to move swiftly to squash the Colorado and Washington reforms. "If they don't act now, these laws will be fully implemented in a matter of months," one of the six, Peter Bensinger, told the Associated Press.

The US government also came under pressure internationally, with a UN body, the International Narcotics Control Board (INCB) warning that creeping decriminalisation and legalisation moves in American states were violating international drug conventions.

But the authors of the Colorado and Washington legalisation laws defended the reforms and said they should be allowed to move ahead. Brian Vicente, co-author of the Colorado law, said his community had sent a message around the world: "You have two states revolting and they're saying it doesn't work in their states," he told the Seattle Times.

Alison Holcomb of the ACLU, who wrote Washington's law, said the DEA chiefs were just stuck in the past. "They're not raising new issues. They're arguing we need to stick with the status quo."

An indication of the kind of issue that is likely to alarm federal law enforcers is the discussion that happened in Colorado on Tuesday among members of the state's task force empowered with regulating the use of marijuana once liberalisation comes in.

The panel recommended that tourists from other states and countries should be allowed to come to Colorado and buy and smoke cannabis freely under the reforms, though the amount they would be allowed to purchase in any one go might be limited to an eighth of an ounce.