The U.S. State Department says it is not aware of talks on Iraqi security that Iranian and Iraqi officials said would resume in Baghdad this week.
State Department spokesman Tom Casey told reporters in Washington Wednesday that neither Iraq nor Iran have informed the United States of their plans.
Iran's ISNA news agency reported Wednesday that a delegation from Tehran has already arrived in Baghdad for a meeting with U.S. officials Thursday.
Iraqi Deputy Prime Minister Barham Saleh also said Baghdad would host a fourth round of discussions this week.
U.S. spokesman Casey said the United States is ready to talk with Iran about Iraq's security situation, but that a meeting would be logistically impossible on Thursday.
He said no meeting has been set since Tehran postponed talks last month.
U.S. and Iranian officials discussed Iraqi security three times in Baghdad last year in the first direct diplomatic contacts between Washington and Tehran in nearly 30 years.
Despite the cooperation, relations between the countries remain tense. Washington accuses Iran of supporting militants in Iraq, which Tehran denies.