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The suicide bombing that killed four U.S. soldiers on Saturday prompted coalition commanders to order their troops to tighten security measures and defenses against guerrilla attacks.

“What it requires is units to conduct force-protection activities,” Maj. Gen. Stanley McChrystal, vice director of operations for the Joint Chiefs of Staff, said Saturday.

“It looks and feels like terrorism,” he said. “Clearly when you see a tactic like this, it requires strict adherence or adjustments to your tactics, techniques and procedures to ensure that places like checkpoints are not vulnerable.”

Some coalition military commanders in Iraq already were instructing soldiers not to allow any vehicles to even approach the checkpoints anymore. The occupants would have to walk.

“Any vehicle that approaches the checkpoint, we’ll have to take appropriate measures,” said Maj. Eric Wick, executive officer of the 2nd Battalion, 70th Armor of the 3rd Infantry Division. Asked whether that meant the approaching vehicle would be fired upon, Wick said yes.

“Saddam is betting that such precautionary measures will alienate the population,” said Shmuel Bar, a senior fellow at Israel’s Institute of Policy and Strategy and an expert on terrorism. “You can’t protect yourselves against suicide bombings by apparent innocent civilians while trying to appeal to the civilian populations.”

Saturday’s bombing occurred just outside Najaf, the world’s leading center of Shiite Muslim theology and the birthplace of the radical Shiite thinking that inspired the Islamic revolution in Iran and the Hezbollah movement in Lebanon, along with many smaller Shiite groups.

The alumni of its religious schools include Iran’s Ayatollah Ruhollah Khomeini and Sheik Mohammed Hussein Fadlallah, whose teachings inspired Lebanon’s Hezbollah movement.

As a center of religious radicalism, Najaf has a long history of rebellion against the secular Baathist regime in Baghdad, which has launched repeated bloody crackdowns against its religious leaders. But its most prominent ayatollahs called last year on Iraqi Shiites to resist any attempt by American forces to enter Iraq.

The Iraqi government’s statement that Saturday’s bomber was an Iraqi soldier, and that there would be more such attacks, suggests that the motive was nationalist and possibly organized by Hussein’s secular regime.

In a television interview two weeks ago, Iraqi Foreign Minister Naji Sabri suggested that the regime had recruited and trained candidates to carry out suicide bombings.

“For many months, tens of thousands have volunteered to serve as martyrdom seekers in the battle with the American enemy,” Sabri said. “We trained them and readied them.”

U.S. Air Force Maj. Gen. Victor Renuart and other commanders said U.S. and British forces would now have to be more careful at roadblocks.

U.S. forces have been using the roadblocks to maintain tight closures around Iraqi cities where they are battling with Fedayeen guerrillas and other irregular Iraqi forces to cut off the enemy’s supplies and communication lines.

Tensions also rose among British troops near Basra. They had been instructed to stop vehicles 50 meters from their checkpoint and tell the occupants to leave their cars and walk toward the soldiers.

Soldiers then would check the occupants and their vehicles separately.

“If they start coming toward us, I’ll empty 200 rounds in them,” said Lance Cpl. Dean Brusby, 24, as he manned a machine gun at a checkpoint on the road to Basra’s airport.