Iran's official news agency says security forces have been called in to seal off the country's 900-kilometer border with Afghanistan.
IRNA news agency says the move is designed to prevent an influx of refugees into Iran, should the United States launch a military strike against Afghanistan.
The U-N refugee agency says Iran already hosts the largest refugee population in the world, with about two million people. At least one-point-four million of those refugees are from Afghanistan.
The United States has denounced Afghanistan for granting sanctuary to Saudi Islamic militant Osama bin Laden, whom Washington is calling a prime suspect in Tuesday's terrorist attacks.
Thousands of Afghans have already fled their capital, Kabul, in anticipation of a U-S military strike. Iran has had strained relations with Afghanistan's ruling Taleban militia.