The US-led authority in Iraq said on Saturday it would introduce a new border policy to try to keep out foreign insurgents, including closing off most of the official crossing points from Iran.
US officials say there is an increasing threat from foreign fighters in Iraq, who they believe are behind some of the major bombings of recent months.
When attacks on Shia taking part in Ashura mourning ceremonies earlier this month killed more than 180 people, Iraqi religious leaders criticised Washington for not doing enough to police borders and protect the country it is occupying.
After those attacks, the US administrator in Iraq Paul Bremer reiterated that Washington had committed $60 million to border security. On Saturday, he issued a statement saying several measures would be accelerated.
"Foreign terrorists are present in Iraq," Bremer said in a statement.
"The numbers are not known with precision, but recent attacks and their continuing presence underscores the importance of improving security at Iraq's borders.
"That is why we are accelerating border security efforts. We will monitor, limit and control the number of people crossing into Iraq from other nations."
Just three of more than 10 official entry points from Iran would remain along the nearly 1,500-km (1,000-mile) frontier. There would be 20 major crossing points into Iraq in total.
Bremer said the number of personnel devoted to border security would be doubled. There are currently about 8,000.
All visitors arriving in Iraq by land will need to present a passport, fill out an entry form, be issued a temporary entry period, and be entered into an computer system.
US authorities in Iraq have indicated the initial focus of border controls would be the Iranian and Syrian borders.
Thousands of Iranian pilgrims crossed into Iraq ahead of the Ashura ceremony, many without official papers.
"We are aware of and have taken into account the importance of religious pilgrims visiting Iraq's holy sites," Bremer's statement said. "Special procedures will be put into place to accommodate increased flows during religious holidays."
US military officials say that while foreign fighters may be involved in some of the larger attacks in Iraq, the majority of insurgents are Iraqi.