Senator Arlen Specter, a Pennsylvania Republican, said on Friday the visit could set the stage for a later mission by US lawmakers.
The senator, who met the Iranian ambassador to the United Nations, Muhammad Javad Zarif, on Wednesday said: "The delegation is confirmed, they are going next month." However, he gave no exact date.
"The Iranian government is not willing to have government to government talks but they feel comfortable with a step at a time," he added.
"They are skittish about going too far and we have gotten to the point where they will accept a small delegation of staffers."
He went on: "I think that will set the stage for meetings with parliamentarians and I think we are laying the groundwork for trying to improve relations with Iran, which would be a big boost."
"The Iranian government is not willing to have government to government talks but they feel comfortable with a step at a time. They are skittish about going too far and we have gotten to the point where they will accept a small delegation of staffers"
Senator Arlen Specter
The United States severed relations with the Islamic government in Iran in 1980 following a crisis over hostages seized from the US embassy in Tehran.
Only two years ago President George Bush said Iran was part of the weapons proliferating "Axis of Evil", along with Iraq and North Korea.
But while the US has expressed concern about Iran's suspected nuclear weapons research, relations have shown signs of a thaw.
The US provided relief assistance to the Iranian city of Bam, devastated by an earthquake in December, and proposed sending a high-level humanitarian delegation to Tehran.
While appreciative of the US earthquake aid, Tehran said the visit of a delegation was best delayed.
Specter was among a group of US lawmakers who met Iran's UN ambassador on Friday.
Dennis Hastert, the leader of the Republican majority in the House of Representatives, was also present.
Specter said the discussions were "fruitful."
He said: "We talked about terrorism, we talked about cooperation against al-Qaida. We talked about their nuclear programme."
The State Department said on Friday that the Bush administration would not oppose any trip by US lawmakers to Iran.
"In general, we've always encouraged people-to-people exchanges with Iran. We certainly encourage congressional travel in general. It sounds like it would be fine with us, if that's what they decide to do"
US State Department spokesman
"In general, we've always encouraged people-to-people exchanges with Iran," spokesman Richard Boucher said.
"We certainly encourage congressional travel in general. It sounds like it would be fine with us, if that's what they decide to do."
A senior State Department official said later that if such a delegation did travel to Iran, the administration would expect the lawmakers to raise US concerns about the country.
In particular, the US is concerned about Iran's support for anti-Israel groups, its opposition to the Middle East peace process, human rights and its alleged pursuit of nuclear weapons.
Iran is opposed to the state of Israel which it accuses of stealing Palestinain land and brutalising its people. And it denies it has a nuclear weapons programme.