A U.S. State Department spokesman says U.S., Iraqi and Iranian officials have postponed plans to meet next week to discuss security in Iraq.
Spokesman Sean McCormack told reporters Thursday that Iran asked to delay the talks because of scheduling conflicts.
U.S. and Iranian officials have met three times this year in Baghdad, but the talks ended inconclusively with each side blaming the other for Iraq's violence.
In another development Thursday, the U.S. military in Iraq said the Iraqi Civil Defense Corps confiscated a cache of Iranian-manufactured rockets last week. A U.S. commander said this is the first time Iraqi personnel have turned over Iranian weapons to coalition forces.
And top U.S. officials in Iraq "strongly condemned" Wednesday's car bombings in the mainly Shi'ite southern city of Amarah that killed at least 40 people and wounded about 125 more.
In a joint statement Thursday, Ambassador Ryan Crocker and the commander of U.S. forces in Iraq, General David Petraeus, also promised to work closely with Iraqi authorities to help bring the attackers to justice.
There has been no claim of responsibility for the three car bombings, targeting a garage, a market and a local police station. Many of those killed were bystanders who had gathered at the site of the first blast.
Iraq's southern Shi'ite-majority areas have been largely free from large-scale attacks. But fighting among rival Shi'ite factions in the region has intensified as the British military hands over security responsibilities to Iraqi authorities.
Some information for this report was provided by AFP.