Iran had made a decision to curtail its support for Shiite factions in 2008 but has now increased its activity in Iraq, sending in lethal arms that were being used against American forces, Admiral
Mike Mullen said Thursady, AFP reported.
"Iran is very directly supporting extremist troops which are killing our troops," Mullen told reporters at a Pentagon Press Association luncheon.
The Iranian shipments are "significant and improved," said the admiral, including a makeshift rocket launching system, known as an improvised rocket assisted mortar (IRAM), and armor-piercing shaped charges, or explosively formed projectile (EFP).
"They are shipping high-tech weapons in there, IRAMs, EFPs, which are killing our people. And the forensics prove that," he said.
As the United States and Iraq negotiate a possible follow-on US force that would stay beyond a December 31 deadline, Mullen said any future security deal with Baghdad would have to take into account the threat posed by neighboring Iran.
"If we reach an agreement (on US troops remaining in Iraq)... it has to be done in conjunction with control of Iran in that regard," he said.
Mullen also said the arms were flowing into Iraq with the full knowledge of Iran's leadership.
"I would say they know about it," he said.
Mullen, chairman of the US Joint Chiefs of Staff, said there was no question that Iran was trying to exert influence in Iraq, especially in the country's south.
His comments echoed the US ambassador in Baghdad, James Jeffrey, who said last week that insurgent Shiite groups beholden to Iran posed a threat to Iraq's stability.
US forces last month suffered their worst casualties in three years with 14 soldiers killed, most in rocket attacks.