The United States is investigating Italian reports that two Arabs carrying explosives arrested in Rome Tuesday arrived aboard an Iraqi airline flight from Baghdad.

Although the State Department had no official comment on the reports, one administration official said, "We don't know anything about them. We are trying to get the facts."

An Iraqi Embassy spokesman said the Italian police reports are "definitely not true."

"We would never allow that," he said, referring to reports that the two Arabs arrived with suitcases loaded with bombs and guns. "Anybody who has been to Baghdad knows that."

He blamed the reports on unnamed elements that he said "might get benefit out of poisoning U.S.-Iraqi relations."

Nonetheless, the reports raised the question of whether Iraq has become involved again in abetting Arab terrorism against U.S. and Israeli targets after several years of strictly forbidding Arab terrorists to operate from Baghdad.

Security at Baghdad airport is known to be extremely tight, and U.S. officials said that, if the reports are true, it seems unlikely that the two Arabs could have boarded an Iraqi plane with guns and explosives without government approval.

Proof that Iraq aided the two Arabs could seriously affect relations between Baghdad and Washington, which have improved steadily since early 1982 when Washington took Iraq off its list of countries supporting international terrorism.

In a letter to Rep. Howard L. Berman (D-Calif.) last June 20, Secretary of State George P. Shultz promised that, should the United States "conclude that any group based in, or supported by, Iraq is engaged in terrorist acts, we would promptly return Iraq to the list of countries identified as supporting terrorism."

Such a determination would reinstitute U.S. trade restrictions on Iraq and severely complicate the rapprochement between the two governments.

Berman had wanted to attach an amendment to the Export Administration Act placing Iraq on the list of countries supporting terrorism but relented on the basis of Shultz's commitment in his letter.

An aide to Berman said he had asked the State Department about the reports from Rome implicating Iraq but has received no reply.

The possibility of Iraqi ties to Arab terrorists was raised in reports that Mohammed Abbas, the fugitive Palestinian accused by the United States of involvement in hijacking the Achille Lauro, was carrying an Iraqi passport.

Abbas' group, the Palestine Liberation Front, has had close ties with Baghdad, although it has recently split into three factions, one of which is based in Damascus.

A State Department spokesman said the United States has had "an exchange of information" with Iraq about Abbas and his background since allegations that he had a role in the hijacking. The spokesman said relations between the two nations remain "pretty good."

No evidence has come to light suggesting that the two Arabs arrested in Rome have any connection to Abbas or his group.

Italian airport officials said one of the two, seized at Rome airport with a 20-pound bomb in his suitcase, had intended to use it against Americans and Israelis. The second Arab was arrested as he got off an airport bus at the central train station. A similar bomb was found in his locker, Italian police said.

Police said both carried Moroccan passports and arrived aboard an Iraqi national airline flight from Baghdad.