The new policy had been formulated in conjunction with the Iraqi Governing Council and Interior Ministry, the official told reporters. Full details would be announced later in the day.

Just three of more than 10 official entry points from Iran would remain along the nearly 1,000-mile frontier.

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.

After near-simultaneous attacks this month killed more than 180 people in Baghdad and Kerbala, the US administrator in Iraq Paul Bremer made a statement reiterating that Washington had committed $60 million to border security.

After those attacks, the top Shia cleric in Iraq criticised the United States for not doing enough to police borders and protect the country it is occupying.

The US authorities in Iraq have indicated that the focus of border controls would be the Iranian and Syrian borders.

Entry points along the country's porous borders will be reduced as part of the scheme, drawn up by the Coalition Provisional Authority (CPA), Iraq's Governing Council and the interior ministry, the official told reporters.

Some 10 to 20 points of entry along the Iraq-Iran border would be cut to three at Muntheriyah, Zurbatiyah and al-Shalamasha, he said.

"We expect these three border points along the Iraq-Iran border to be fully functioning points of entry within approximately six weeks," the coalition official said.

"These are part of our broader effort to more aggressively regulate and in some cases limit border traffic coming across the borders," he continued.

It marks a "small but important part of our overall strategy to address issues relating to foreign fighters coming into the country."

US officials say extremists and insurgents, backed by foreign fighters, have now replaced former members of the Baath party and Saddam Hussein loyalists as the biggest security threat in Iraq.

Iraqi Shiite religious leaders blamed US forces for not policing the country's borders tightly enough after the bombings at Shiite shrines in two cities on March 2, in which more than 170 people died and hundreds more were injured.

It was feared that some foreign fighters may have slipped across the border from Iran with thousands of pilgrims who travelled to mark a major Shiite religious holiday in public for the first time in decades.

Meanwhile, Iraq's Shia Muslim clerics criticised the new interim constitution Friday but stopped short of calling for protests, as more US troops died amid warnings that an increase in spectacular attacks was likely. Religious leaders in the holy cities of Karbala and Najaf, where Iraq's most influential cleric, Grand Ayatollah Ali al-Sistani, is based, reiterated their concerns over the text, which they fear gives too much power to the Kurds.

In Najaf, 160km from Baghdad, leader Sadreddin Kubanji criticised the "weakness" of the document and said the power it gives Kurds "threatens the unity of the country."

Calls from religious leaders have multiplied for Shia followers, who make up more than 60 percent of the population, to make their voices heard since Monday's controversial signing of the new basic law.