A national conference viewed as a crucial step in Iraq's postwar democratic development will open on Saturday, two days later than expected, despite a flurry of last-minute problems that could still jeopardize its success.

In the latest threat by Islamic militants, meanwhile, four masked men calling themselves the Group of Death said in a televised videotape that they would cut off the highway to Jordan, Iraq's major trade and supply route, unless Jordan's government stopped cooperating with U.S.-led military forces here.

The warning, broadcast on the al-Arabiya satellite television network, came as a Jordanian company decided to halt construction work at a U.S. military base in Iraq in hopes of saving two kidnapped Jordanian drivers. In the past several weeks, there have been a rash of kidnappings in Iraq, with foreign workers seized and an Egyptian diplomat taken hostage and released.

The national conference chairman, Fuad Masoum, said U.N. officials had asked to delay the gathering, in which 1,000 delegates from a range of political, ethnic and religious groups are scheduled to participate. The delegates are to choose 100 members to form an assembly that will oversee Iraq's interim government until national elections are held next year. Neither the delegate list nor the conference site has been announced.

Masoum said organizers were determined to start the conference by the end of July to comply with the legal framework set up in June by the outgoing U.S. occupation authority. "Any delay would be seen as negative, after living in decades with temporary institutions and constitutions, promises made and never kept, delays and postponements," Masoum told reporters. "We didn't want to leave any doubts."

Still, there were numerous signs that final preparations for the meeting had run into serious difficulties, including the collapse of the formal nominating process for delegates in several provinces and the refusal by important religious groups to participate in what they said was an American-staged sham.

Masoum said that in Kirkuk, a multiethnic city in northern Iraq, "difficulties between different ethnic groups" had forced officials to annul the delegate-selection process this week and that conference officials would now meet with local leaders. Sources in Kirkuk, an oil production hub coveted by ethnic Kurds, Arabs and Turkmens, said the Kurds and Turkmens could not agree on how to divide the delegate seats.

"The Kurds wanted to have 20 people -- five Kurds, five Christians, five Arabs and five Turkmens -- but that's unfair, because Kirkuk is a majority-Turkmen city," complained Songul Chapouk, a Turkmen leader and member of the now-disbanded Iraqi Governing Council, who spoke by telephone from Kirkuk. "They were just trying to make a mess, and now we have to try and solve the problem tomorrow."

Masoum said there had been other disputes over nominations in various parts of the country. But although the nominating committees had to be scrapped in several cases, he said, the "misdeeds" of certain groups had not been serious enough to necessitate new elections for the conference.

A second and potentially more significant snag was the rejection of the conference by several key religious groups. Iraqi officials had intended the gathering to be as representative as possible, extending to groups that oppose the current government and the U.S. presence.

But representatives of Moqtada Sadr, a popular radical Shiite Muslim cleric, said his group refused to take part in conference nominations last weekend in Sadr's home city of Najaf and that it would boycott the meeting because of disagreement with the way delegates were chosen.

At the same time, leaders of the Association of Muslim Scholars, the most prominent organization of Sunni clerics, said they would not participate because they viewed the conference as an American creation. Sheik Abdul Sattar Abdul Jabbar, a council member, said the group had been "invited to participate, but we refused, because we reject anything that is the result of the occupation."

Masoum stressed that conference leaders had extended invitations to groups representing "all major trends," including Sadr's movement. "We have extended invitations, and whoever attends, we welcome them," he said. "If they have other ideas, we respect them. . . . There is no exclusion of anyone."

At a separate appearance in the capital, Iraq's interim president, Ghazi Yawar, said the assembly chosen at the conference would have an important legislative function. "We truly hope it will represent all Iraqi trends and political powers, even if they don't agree with the government," he said. "I hope every Iraqi will be represented."

Yawar, who spoke several hours before Masoum's announcement, said he saw no reason not to delay the conference further if necessary. The July deadline, he said, "is not a holy thing. If we see the benefit to the nation of changing the deadline, we will do it."

Iraqi officials suggested that planning for the conference would have gone more smoothly if the United Nations had been able to play a larger role. In Afghanistan, two national conferences have been held in the past two years with intensive U.N. oversight, but in Iraq, U.N. activities have been severely curtailed by violence, especially the August 2003 bombing of U.N. headquarters here in which 22 people died.

"We can't do this without the U.N. It would be better if they had come before, but they are limited by their conditions," Masoum said.

Correspondent Doug Struck and special correspondent Omar Fekeiki contributed to this report.

Iraqi driver Hussein Ali, 36, was injured when his truck was blown up north of Baghdad. A group of militants threatened on a televised videotape to cut off the main route to Jordan. Fuad Masoum, chairman of the conference.