A customs dispute between the Afghan and US governments has disrupted the withdrawal of American military equipment, dramatically inflating the cost of the drawdown, defense officials said.

With Afghan authorities insisting the United States owes millions of dollars in customs fines and trucks carrying hardware being blocked at border crossings, the Americans have started flying out most equipment by air at great cost.

"The cost is five to seven times more" by aircraft than over land through neighboring Pakistan, a defense official, who spoke on condition of anonymity, told AFP.

The Afghan government is insisting that US forces pay $1,000 for each shipping container leaving the country that lacks what it calls a valid customs form.

And authorities now claim the Americans owe $70 million in fines, even though the United States contends that Kabul's stance contradicts previous agreements, said US officials, confirming a report that first appeared in the Washington Post.

The US Congress was warned in May by the special inspector general for Afghanistan reconstruction that Kabul was exacting exorbitant customs fees and taxes in violation of previous agreements regulating imported goods and the status of US forces.

In some cases, Afghan officials were blocking commercial trucks from delivering food and fuel to US forces due to the customs dispute, the inspector general said.

Afghan authorities, however, claim that US contractors transporting equipment failed to file proper documents for large amounts of gear shipped into the country since 2010.

To collect fines that Washington allegedly owes Kabul, "the only way is to stop all trucks from crossing the border," Najibullah Wardak, the director general of the Afghan Customs Department, told the Washington Post.

"What else can we do?"

The Pentagon in a statement acknowledged "challenges" with the withdrawal at Afghan border crossings.

The disputes are "typically centered on the interpretation of Afghan customs processes," spokesman Lieutenant Colonel Bill Speaks said in the statement.

NATO-led force commanders are talking with Afghan officials and "we are confident that the situation will be resolved soon," he said.

With a massive drawdown of military equipment due between now and the end of 2014, US officers had counted on moving most of the equipment by land routes through Pakistan.

Recent tensions with Pakistan have previously forced the US and its NATO allies to transport large amounts of cargo by air and by a longer, more expensive route through Central Asia.

Over the past month, only 36 percent of US equipment has been driven out by land routes, the Post wrote, citing a Pentagon official.

The disagreement over customs fines threatens to undermine difficult negotiations over a possible future US military presence after 2014. ddl/adm