General John Abizaid said the weapon, an RPG-29, has a dual warhead and has proved effective against most types of armored vehicles.
"The first time we saw it was not in Iraq. We saw it in Lebanon. So to me it indicates, number one, an Iranian connection," he told defense reporters here.
"It’s hard to say in our part of the world that we operate in as to whether or not people have given us a hint about things to come," he said.
He said only a single RPG-29 has turned up in Iraq so far, and it was unclear how it was smuggled into the country.
But he said it was the latest in a number of new and more sophisticated weapons that appear to be moving onto the region’s battlefields from Iran.
He said longer-range Chinese rockets that looked new also have been found in Iraq.
Abizaid said he believed the Chinese rockets came from Iran although they may have been taken from the arms inventories of the former Iraqi regime and cleaned up. "It looked brand new to us," he said.
The new weapons are in addition to more sophisticated roadside bombs with explosively shaped charges that the US military has long charged are being manufactured in Iran and brought into the country by Iran’s Revolutionary Guards-Quds Force.
Abizaid pointed out that Hezbollah, which is backed by Iran, used a panoply of new weapons against the Israelis in Lebanon.
They included longer range surface-to-surface missiles, an anti-ship cruise missile, new anti-tank weapons that proved effective against Israeli armor, and remotely piloted vehicles packed with explosives, he said.
"There are clearly links between Lebanese Hezbollah training people in Iran to operate in Lebanon, and also training people in Iran that are Shia splinter groups that could operate against us in Iraq," he said.
"These linkages exist but it is very, very hard to pin down with precision," he said.