Iran ordered its border with Afghanistan closed Saturday amid concerns its neighbor could be the target of U.S. retaliation for Tuesday's terror attacks on New York and Washington.

The Islamic Republic News Agency reported that Iran would post additional military and police forces along its 562-mile (900-kilometer) border to prevent Afghan refugees from entering the country. The Interior Ministry said officials in its border provinces should continue cooperating with international relief organizations trying to help Afghans already in Iran. Iran already houses nearly 2 million refugees displaced by two decades of war and famine in Afghanistan. Afghanistan is considered a possible target for any retaliation because Islamic militant leader Osama bin Laden reportedly operates from the Afghan mountains. President Bush on Saturday branded bin Laden as the "prime suspect" in the attacks. Bush has repeatedly threatened to strike out against terrorism and any nation that supports or harbors terrorists, and large numbers of Afghans are reportedly fleeing the country in fear of potential U.S. retribution. In a rare 15-minute radio address Friday night, Mullah Mohammed Omar -- the Taliban's supreme leader -- urged Afghans not to fear a U.S. attack and to put their faith in Allah. He noted that Afghanistan had defeated British and Russian invaders before. "Now, the third empire of the world wants to impose an attack on us," he said. "As you know better, it is not because of Osama. This is the demonization of Islam."