Image caption Medical use of marijuana is already legal in many US states

A group of ex-US drug enforcement chiefs say Washington must crack down on marijuana use in two states that recently voted to legalise the drug.

Eight former Drug Enforcement Administration (DEA) leaders urged the Obama administration to sue the states.

In November, Colorado and Washington state voters legalised marijuana, which is still banned under federal law.

Analysts say targeting recreational users is not a top enforcement priority for the US justice department.

In a letter released to the news media, the former DEA chiefs say President Barack Obama's justice department must act swiftly to counteract the state laws.

And they called on senators to ask Attorney General Eric Holder, an Obama appointee, why the federal government had not already taken action when he testifies in Congress on Wednesday.

Under the US Constitution, federal law trumps state law, and the US is bound by international treaty to keep cannabis illegal, argue the former DEA chiefs, who worked under both Democratic and Republican presidents.

In the two states where voters have legalised recreational cannabis use, "the process of writing regulations is underway and it will become harder to rescind the legislation," Peter Bensinger, who ran the DEA under Presidents Gerald Ford, Jimmy Carter and Ronald Reagan, told the BBC.

'Scare tactics'

He added the Obama administration should sue Colorado and Washington state as it recently had the state of Arizona over an immigration measure it deemed clashed with federal law.

Mr Bensinger warned that full implementation of the legalisation laws would "cause havoc" in the communities, schools and roads of Colorado and Washington state.

"The highways would be littered with fatalities because you're going to have increased use and marijuana stays in the body much longer than alcohol," Mr Bensinger said.

But Sanho Tree, a drug policy analyst who supports legalisation, said the former DEA chiefs' warnings were "scare tactics".

Their letter is an attempt to goad the Obama administration into a show-down with the states in the Supreme Court, said Mr Tree, director of the drug policy project at the Institute for Policy Studies.

"Most of these drug policy statements are about turf, which is really about budgets," said Mr Tree. He said drug enforcement agencies were attempting to "maintain a monopoly on doctrine".

He said the federal government must be realistic about changing attitudes towards the drug, and he said that in states with medical marijuana laws in place, critics' worst fears had not come to pass.