Arab and Iraqi authorities speaking to London's pan-Arab daily Asharq al-Awsat said they uncovered an "elaborate plot" to kill Allawi, the leader of the secular Iraqiya slate.
Sources told the news agency that Iraqi intelligence officials in cooperation with Arab neighbors tracked down several suspects in the plot and seized weapons allegedly linked to the plans.
"U.S. forces operating in Iraq verified this information and helped disclose it," the source told the news agency on condition of anonymity. "Local groups" and "regional parties" were behind the plot.
The source added the plot was to be carried out before Allawi started making plans to form the next government.
Iraqiya won 90 seats in the March 7 election for the 325-member Council of Representatives, beating Iraqi Prime Minister Nouri al-Maliki's State of Law coalition by two seats.
A 163-seat majority is needed to unilaterally form a new government.
State of Law announced Thursday it formed a grand Shiite coalition with the Iraqi National Alliance that is four seats short of the majority.
The alliance was formed following talks in Najaf between leaders of the Supreme Islamic Iraqi Council and Grand Ayatollah Ali Sistani, the Voices of Iraq news agency.
The Shiite alliance has not settled on a candidate for prime minister, however.
Parliament is scheduled for its first post-election session Monday.