The U.S. military has evidence that Shiite extremists in Iraq are receiving arms and training from Iran, but it is not clear if the Iranian government is involved, a U.S. spokesman said Monday.

"We know that some Shiite elements have been in Iran receiving training," U.S. military spokesman Maj. Gen. William B. Caldwell told reporters. "But the degree to which it is known and endorsed by the government of Iran is uncertain."

The Iranian government did not immediately respond to the comments.

"We do know that weapons have been provided and IED technology been made available to these extremist elements," Caldwell said, referring to improvised explosive devices, or homemade bombs, that are in widespread use in Iraq's insurgency and sectarian conflict.

Caldwell's assertion came on the heels of allegations by U.S. Ambassador Zalmay Khalilzad, who told The New York Times that Iran was encouraging Shiite militias to step up attacks on U.S. forces in retaliation for the Israeli assault on Hezbollah in Lebanon. The Shiite Hezbollah is backed by Iran.

Iran's prodding has led to a surge in mortar and rocket attacks on the fortified Green Zone that houses the main components of the Iraqi government and the U.S. Embassy.

Four Australian soldiers were wounded Monday in a rocket attack on the Green Zone. Three were treated and released but a fourth soldier, a woman, suffered serious head and internal injuries, Australia's military chief said.

Caldwell did not refer to Khalilzad's allegations but said the weapons used by Shiite extremists here had markings that made it clear they were manufactured in Iran within the last three years.

"This is not to say that the government of Iran is associated with that, but it did come from Iran, the munitions and the weapons," he said.

Caldwell refused to say if the main Shiite militia, the Mahdi Army of anti-U.S. cleric Muqtada al-Sadr, was involved in attacks on U.S. forces or had received arms and training from Iran.

"Right now the only way we can define it is they are Shiite extremist elements. So it's really the extremist elements out there that are the ones ... involved with training and receiving arms and ammunitions. we are not attributing it to any group in Iraq itself," he said.