BAGHDAD -- Iraq closed its borders with Iran and Syria as U.S. and Iraqi troops tightened their grip on Baghdad on Thursday, setting up more checkpoints that stopped and searched even official convoys for weapons.

An Interior Ministry official said the closure of Iraq's four border crossings with Iran and two with Syria took effect Wednesday.

U.S. officials have long accused Syria of allowing foreign fighters to cross its long, porous border into Iraq, and over the weekend presented evidence of what they said were Iranian-manufactured weapons being smuggled into Iraq.

"The plan to close the borders went into effect last night. Many points were closed, but I can't confirm that all were shut," U.S. military spokesman Lieutenant Colonel Christopher Garver said.

There was no immediate confirmation from Tehran or Damascus, both of which deny involvement in Iraq's chaos.

Iraq had said it would shut the borders for 72 hours. The U.S. military said Wednesday that the aim was to choke off the flow of weapons and foreign fighters into the country.

The closures came as thousands of U.S. and Iraqi troops stepped up an offensive in Baghdad, the epicenter of sectarian violence between minority Sunnis and majority Shiites, which has pitched the country toward all-out civil war.

Military analysts say many militiamen are likely to lie low or leave Baghdad until the operation is completed rather than seek confrontation with U.S. and Iraqi forces.