By Jon Hemming

KABUL, July 17 (Reuters) - Iranian arms are entering Afghanistan and reaching Taliban insurgents in such quality and quantity that the Tehran government must know about it, the U.S. ambassador to Kabul said on Tuesday.

The charge is similar to that made by U.S. Defense Secretary Robert Gates a month ago, rejected by Tehran as "baseless and illogical".

But it comes as Iran signalled on Tuesday there was a "high possibility" it would hold a second round of talks with the United States on Iraqi security in the "near future".

"There are clearly some munitions coming out of Iran going into the hands of the Taliban," said Ambassador William Wood.

"We believe that the quantity and quality of those munitions are such that the Iranian government must know about it," he told reporters. "Beyond that we really can't go."

The United States accuses Iran of stoking instability in the Middle East by arming insurgents in Iraq, aiding militants in Lebanon and Gaza and trying to build a nuclear arsenal.

Iran denies the charges and says the U.S. military presence in Iraq and Afghanistan, and Washington's support for Israel is the source of instability in the Middle East.

U.S. officials said for several months this year they had evidence of Iranian weapons entering Afghanistan, but until Gates spoke a month ago, stopped short of linking the arms to the Iranian government.

Afghan President Hamid Karzai has said relations with Iran have never been better and said his government does not have any evidence of Iranian arms reaching the Taliban.

Afghanistan has seen a rise in Taliban suicide and roadside bombs in the last two years, but the 50,000 NATO and U.S.-led forces say they have largely thwarted a much-heralded Taliban spring offensive.

"For the most part what we have seen has been terroristic violence rather than insurgent violence," said Wood. "The security situation is better, but it doesn't feel better."

Iran was at odds with the Taliban government for most of the time it held sway in neighbouring Afghanistan from 1996 to 2001 and massed troops on the Afghan border on 1998 after 11 of its diplomats were killed there.

It then armed Northern Alliance factions that helped U.S.-led forces overthrow the Taliban after the Sept. 11, 2001 attacks.

But political and security analysts based in Kabul say Iran may have an interest in destabilising Afghanistan so as to discredit U.S. attempts to foster democracy there.

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