The United States is worried that Iran may be backing Afghan fighters in an attempt to unsettle the U.N.-brokered government in Afghanistan, the U.S. envoy here said Friday.

Zalmay Khalilzad's comments echoed a warning last week by President Bush, who accused Iran of interfering in neighboring Afghanistan and harboring fighters from Usama bin Laden's Al Qaeda terror network. Iran denied Bush's allegations.

Khalilzad cited reported that Iran may be sending pro-Iranian Afghan fighters and money into Afghanistan and encouraging opposition to the administration of Hamid Karzai, which was inaugurated in December after the fall of the Taliban militia and mandated to rule for six months.

"All of those things would be regarded as interference," Khalilzad said. He stopped short of saying Iran's involvement was a certainty, but said it was a "fair assumption."

The criticism of Iran — which denies trying to destabilize the Afghan government — came as Karzai left Afghanistan on his first trip abroad since his inauguration, aiming to drum up money for a government that is flat broke and faced with rebuilding the war-torn nation.

After a stop in Mecca, Saudi Arabia, to perform a Muslim pilgrimage, Karzai heads to Tokyo for a conference of donor nations aimed at raising an initial $5 billion in aid. Karzai will return home, then head off again to Washington for a Jan. 28 meeting with President Bush, said an aide, known by the single name Humayan.

Until recently, the United States had quietly praised longtime foe Iran for its help in the war on international terror. But on Jan. 10, Bush accused Iran of harboring Al Qaeda fighter and warned Tehran not to try to destabilize Karzai's government. Khalilzad did not mention the Al Qaeda allegations.

Iran — which was an opponent of the Taliban regime that harbored Al Qaeda — denied Bush's claims. "It has been our policy not to allow terrorist groups such as Al Qaeda in Iran," Iranian Vice President Mohammad Ali Abtahi said.

Afghan Defense Minister Gen. Mohammed Fahim was quoted Wednesday by Iranian state radio saying there was no sign of Iran "creating insecurity" in Afghanistan.

Iran has long had influence in western Afghanistan. Some analysts have said it may be working with local warlords to safeguard its interests — something Pakistan and Russia have also done.

As Karzai departed from the Afghan capital, Kabul, his interim government said it needed the world's help.

"We pledged to the international community ... that we would bring change in our country," Interior Minister Younus Qanooni said. "Now it is their turn to pledge their help ... If they will not help and assist us, we will face many problems."

The Tokyo conference, taking place on Monday and Tuesday with some 50 nations attending, aims to raise $5 billion dollars for the first 2 1/2 years of reconstruction.