Mohammad Ali Hosseini's comments come a day after Iranian diplomats met with their detained countrymen in Iraq for the first time.
"The five (detained) diplomats during the meeting were complaining about the Americans regularly breaking promises for their release," Hosseini said during his weekly news briefing. "This was one of the psychological pressures that they complained about."
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Hosseini did not provide additional examples of psychological pressure, and it was unclear to which promises he was referring. The U.S. has not publicly promised to release the five Iranians, who were detained on Jan. 11 in the northern Iraqi city of Irbil.
U.S. authorities have said the five included the operations chief and other members of Iran's elite Quds Force, which is accused of arming and training Iraqi militants.
Hosseini said Sunday that the detainees considered the U.S. government's claims "false and baseless." Iran has consistently denied the U.S. allegations and insists the five are diplomats in Iraq with permission of the government.
The detention has been a point of contention between Tehran and Washington at a time when the Iraqi government is trying to get the two to resolve their differences.
On Saturday, Iran's ambassador to Iraq, Hassan Kazemi Qomi, and his staff made their first visit to the five detainees and repeated calls that they be immediately released.
The U.S. military said the visit by the three Iranian diplomats lasted several hours and "took place at an MNF-I (Multi-National Forces — Iraq) detention facility in Iraq."
In a statement to The Associated Press, it said the visit followed "standard procedures" for visits by foreign governments to their nationals, but it did not specify which facility the five were held at. The U.S. military oversees some 21,000 inmates at Camp Bucca in southern Iraq and Camp Cropper, near the Baghdad airport.
Iran's foreign ministry spokesman said the five Iranians had complained of limited access to facilities during detention, but did not elaborate.
However, Qomi indicated Saturday there had been "an improvement in terms of their detention situation."
"Their situation is different than the first time they were detained, they are staying today in a special room, they can be together at particular hours during the day, they spend part of their time doing sports and other activities," Qomi told the country's Arabic language Al-Alam television channel in a phone interview.
Qomi told Iran's official news agency IRNA from Baghdad on Sunday that the diplomats would be allowed to meet with their relatives "in the coming days." He said the International Committee of the Red Cross already had facilitated phone conversations between the diplomats and their families.
The Iranian ambassador also told IRNA that two additional Iranians, Majid Dagheri and Heidar Alavi, were being detained by the U.S. in the same prison.
Qomi said the men "were abducted by American forces one and three years ago respectively in Suleimaniyah in eastern Iraq," some 30 miles from the Iranian border. He identified the men as bank employees Saturday during his interview with Al-Alam.
There has been no U.S. confirmation of other Iranians being held.
The Iraqi government, which is backed by the U.S. but closely allied to Iran, has been trying to get the two sides together, hoping some cooperation will reduce violence in the country.
Iraqi Foreign Minister Hoshyar Zebari said he hoped the visit Saturday to the detainees would help ease tensions between Iran and the U.S.
The visit came a week after Iran said it would consider "with a positive point of view" an Iraqi request for a new round of Iranian-American talks, but only after the U.S. responds to the invitation.
But Hosseini said Sunday that Saturday's meeting and Iran-U.S. talks were "separate issues."
The U.S. and Iran held groundbreaking ambassador-level discussions on May 28 in Baghdad to address security in Iraq.
But since then, bitterness has mounted further between them, partly due to Tehran's detention of four Iranian-American scholars and activists charged with endangering national security. The U.S. has demanded their release, saying the charges against them are false.
Complete coverage is available in FOXNews.com's Iraq Center.