BAGHDAD, Iraq — The U.S. ambassador to Iraq warned Iraqi politicians today they risk a loss of American support if they do not establish a genuine national unity government, saying the United States will not invest its resources in institutions run by sectarians.

Meanwhile, at least 24 people, including an American soldier, were killed by bombings in Baghdad and elsewhere. Two Macedonian contractors were freed by kidnappers four days after they were abducted in Basra, a British official said without giving further details.

U.S. Ambassador Zalmay Khalilzad delivered his blunt warning during a rare press conference after signs that talks on a new government following the December elections were not going well because of sharp differences among the country's Shiite, Sunni Arab and Kurdish political parties.

Failure to establish a unity government that includes a strong role for Sunni Arabs would fail to undermine the Sunni-dominated insurgency and throw into question U.S. plans for a phased withdrawal of the 138,000 American troops here.

The ambassador reminded the Iraqis that the United States has spent billions to build up Iraq's police and army and "we are not going to invest the resources of the American people and build forces that are run by people who are sectarian."

Sunni Arabs accuse the Shiite-led Interior Ministry of human rights abuses and using Shiite militias against Sunni civilians under the cover of fighting the insurgency. Shiites deny the charge and say they must control security forces to protect Shiites against attacks by Sunni religious extremists.

Khalilzad cited the need for compromise, especially in the Defense and Interior ministries. He said ministers in those posts must be those "who are nonsectarian, broadly accepted and who are not tied to militias."

Otherwise, he warned that "Iraq faces the risk of warlordism that Afghanistan went through for a period." Khalilzad was born in Afghanistan and served as U.S. envoy there.

Several Shiite parties are believed to control armed militias, some of which date back to the Iran-Iraq war of the 1980s when many Iraqi Shiites fled to Shiite-dominated Iran. On the other hand, most of the insurgents are Sunnis. Kurds maintain the biggest armed force — the peshmerga — which they maintain is the legitimate security force of their autonomous government in the north.

Iraqis have until mid-May to form a new government, but U.S. and Iraqi officials warn the process could take longer because of political differences.

Shiites, who comprise about 60 percent of the population, are reluctant to surrender power won at the ballot box to Sunni Arabs, who dominated political life here during Saddam Hussein's regime. Sunni Arabs insist that programs to purge Saddam's supporters from public life be limited and not used to deprive Sunnis of a future in Iraq.

Kurds zealously guard the self-rule they have enjoyed since 1991, and many of them want to expand their autonomous region to oil centers around Kirkuk, claimed by Kurds, Arabs and Turkomen.