By Andrew Gray

RAMSTEIN AIR BASE, Germany, June 13 (Reuters) - Iranian weapons are entering Afghanistan on such a scale that it is hard to believe Iran's government is not aware of the movement, U.S. Defense Secretary Robert Gates said on Wednesday.

U.S. officials have said for several months that they have evidence of Iranian weapons entering Afghanistan but have stopped short of linking the arms to the Iranian government.

But Gates said he had seen more information since the original evidence was uncovered.

"I have seen additional analysis in the interval that makes it pretty clear there's a fairly substantial flow of weapons," he said on a visit to U.S. military bases in Germany.

"Given the quantities that we're seeing, it is difficult to believe that it's associated with smuggling or the drug business or that it's taking place without the knowledge of the Iranian government," he told reporters at Ramstein Air Base in western Germany.

His comments came the day after U.S. Undersecretary of State Nicholas Burns openly accused Tehran of arming Taliban insurgents in Afghanistan.

The U.S. accusations -- which Iran denies -- add another source of tension to the testy relationship between Washington and Tehran.

The two countries are at loggerheads over Iran's nuclear programme and Washington has also accused Iran of arming Shi'ite militants in Iraq.

Gates said he believed the Iranian weapons intercepted in Afghanistan were intended for Taliban insurgents. He did not specify how many Iranian weapons were entering Afghanistan.

Asked if the weapons included the armour-piercing explosively formed projectiles which have proved particularly deadly in Iraq, Gates said a range of weapons was involved but did not elaborate.

He said it was ironic that the Afghan government enjoyed good relations with its Iranian counterpart.

"Whether Iran is trying to play both sides of the street or hedge their bets, what their motives are -- other than causing trouble for us -- I don't know," Gates said.

The United States has around 26,000 troops in Afghanistan, most of them part of a NATO security force and others engaged in separate missions from training Afghan forces to counterterrorism operations.

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