BAGHDAD - U.S. officials say border security will be tightened following Tuesday's terror attacks on Shi'ite worshipers in Baghdad and Karbala. Iraq's Governing Council says the signing of Iraq's interim constitution, postponed to the end of a three-day mourning period for the victims, will take place Friday. The Council puts the death toll of Tuesday's attacks at 271, sharply higher than U.S. coalition estimates.

The top U.S. administrator in Iraq, Paul Bremer, blamed foreign terrorists, naming al-Qaida operative Abu Musad al Zarqawi, for trying to ignite civil war and disrupt Iraq's efforts to establish a stable democratic government. "In spite of the death toll, in spite of the wounded, as Iraqis across the political spectrum have made clear in their statements, efforts to provoke sectarian violence have failed and will fail," he said.

Mr. Bremer says the U.S. administration will contribute $60 million to increase border security. He says police forces in selected areas would be doubled and more equipment would be added. There are currently eight thousand border police on the job.

As a sign the violence has not abated, mortars were fired late Wednesday into the so-called Green Zone in central Baghdad where the U.S. coalition forces are headquartered. The attack delayed Mr. Bremer's news briefing.

Iraqi police on Wednesday said they have apprehended 15 suspects in connection with the bombings in Baghdad and Karbala.

Earlier in the day mourners began the grim task of burying victims of the blasts. They carried coffins, draped with Iraqi flags and traditional black banners of mourning, in front of the same shrines in Baghdad and Karbala where tragedy had struck just one day before.