Iranian forces have battled for years in the lonely canyons and deserts on the Afghan border against opium and heroin traffickers — winning rare praise from the United States and aid from Europe for the fight along one of the world's busiest drug routes.

But now, international support for Iran's drug agents could be threatened by the standoff over Tehran's nuclear policies.

Western nations have told Iran that they could cut off any new help to Iran's anti-drug units unless the Islamic regime halts uranium enrichment, which Washington and its allies worry could be used to develop nuclear arms.

The warning was a small but potentially significant item tucked amid an array of trade and economic incentives seeking to sway Iranian leaders to strike a deal. Iran has not formally responded to the package, presented June 14 by the five permanent United Nations Security Council members plus Germany.

But Iran has repeatedly said it will not back off uranium enrichment — pushing the European Union to consider wider sanctions and placing the little-known drug trafficking fight in jeopardy.

It's the type of pressure that strikes the United Nations as counter-productive.

A "heroin tsunami" could hit Europe if the drug interdiction by Iran is weakened, warned Antonio Maria Costa, the director of the U.N. Office on Drugs and Crime.

"We should definitely assist Iran in this respect," he said.

Roberto Arbitrio, head of the U.N. drugs and crime office in Iran, said the war on drugs should be viewed as "a non-political area of mutual interest."

The new stance is a sharp departure from the strong — but mostly behind-the-scenes — cooperation the United States and other Western countries forged with Iran on Afghanistan after the Taliban's fall in late 2001.

The West and Iran shared a common enemy in the Taliban, the Sunni extremist group that gave shelter to al-Qaida leader Osama bin Laden before the Sept. 11, 2001 attacks and now continues to fight the U.S. military and NATO.

Taliban fighters help finance their battles by taxing Afghanistan's opium farmers, whose poppies provide the raw material for heroin. The West has had little success reducing the huge opium crop in southern Afghanistan where the Taliban is strongest.

Overall opium production in Afghanistan has more than doubled in the last four years — and smuggling the drug into Iran is the first step toward reaching Western markets. Afghanistan produced 93 percent of the world's opium last year, and about 50 percent of the drugs leaving the country flowed through Iran, the U.N. Office on Drugs and Crime says.

"Cooperating with Iran in Afghanistan on this and other issues is not a favor we do for Iran — but something we need to do in our own interest," said Barnett Rubin, an expert on Afghanistan at New York University.

The incentive package promised Iran "intensified cooperation in the fight against drug trafficking" from Afghanistan — but only if it suspends uranium enrichment first. Iran claims its nuclear program is only for energy producing reactors and insists it has the right to have uranium enrichment technology.

White House and State Department officials have refused to comment on how halting aid to Iranian anti-drug units might affect the flow of drugs from Afghanistan or the fight against the Taliban.

Washington has recently accused Iran of providing support to the Taliban in order to bog down Western militaries in Afghanistan, although it has offered little public evidence. Iran denies the charge.

The office of EU foreign policy chief Javier Solana, who offered the Iran incentives, also refused comment on the new anti-drug link.

"Fighting drug trafficking should not be politicized," said Ismail Ahmadi Moghaddam, the top anti-drug official in Iran. "When narcotics reach Europe, it is the people, not governments, that suffer."

Establishing security and delivering aid in southern Afghanistan would do much more to tackle the drug problem and stop the Taliban, said Anthony Cordesman, a security analyst for the U.S.-based Center for Strategic and International Studies.

The United States has spent $878 million since 2001 trying to wean Afghan farmers off growing opium — even as production has skyrocketed. Washington also has praised Iran's anti-drug steps.

Iran has built a series of dikes and trenches along large portions of its roughly 560-mile border with Afghanistan to stop drug smugglers and has seized hundreds of tons of opium and heroin. Moghaddam said 900 tons of narcotics were seized last year, including what the U.N. estimated was 80 percent of total world opium seized.

The efforts have taken their toll: More than 3,500 Iranian law enforcement officers have died in clashes with heavily armed drug traffickers over the last two decades, the Iranian government says.

"There is overwhelming evidence of Iran's strong commitment to keep drugs leaving Afghanistan from reaching its citizens," said the U.S. State Department in its 2007 narcotics report on Iran.

Despite that praise, the United States does not donate money to the U.N. to support Iran's anti-drug efforts because of unilateral sanctions. The United Nations, however, has received contributions from several European nations, including Britain, France and Italy, to aid Iran's drug-fighting efforts.

But political disputes have made fundraising to help Iran difficult, Arbitrio said. His office has raised only $8.5 million since 2005 for a three-year program originally budgeted at $20 million to help Iran intercept narcotics smuggled from Afghanistan and other measures.

"Iran is a front-line country," said Costa of the United Nations.