BAGHDAD, Jan. 28 — Iran’s ambassador to Baghdad outlined an ambitious plan on Sunday to greatly expand its economic and military ties with Iraq — including an Iranian national bank branch in the heart of the capital — just as the Bush administration has been warning the Iranians to stop meddling in Iraqi affairs.

Iran’s plan, as outlined by the ambassador, carries the potential to bring Iran into further conflict here with the United States, which has detained a number of Iranian operatives in recent weeks and says it has proof of Iranian complicity in attacks on American and Iraqi forces.

The ambassador, Hassan Kazemi Qumi, said Iran was prepared to offer Iraq government forces training, equipment and advisers for what he called “the security fight.” In the economic area, Mr. Qumi said, Iran was ready to assume major responsibility for Iraq reconstruction, an area of failure on the part of the United States since American-led forces overthrew Saddam Hussein nearly four years ago.

“We have experience of reconstruction after war,” Mr. Qumi said, referring to the Iran-Iraq war in the 1980s. “We are ready to transfer this experience in terms of reconstruction to the Iraqis.”

Mr. Qumi also acknowledged, for the first time, that two Iranians seized and later released by American forces last month were security officials, as the United States had claimed. But he said that they were engaged in legitimate discussions with the Iraqi government and should not have been detained.

Mr. Qumi’s remarks, in a 90-minute interview over tea and large pistachio nuts at the Iranian Embassy here, amounted to the most authoritative and substantive response the Iranians have made yet to increasingly belligerent accusations by the Bush administration that Iran is acting against American interests in Iraq.

President Bush has said the American military is authorized to take whatever action necessary against Iranians in Iraq found to be engaged in actions deemed hostile.

The Iranian ambassador abruptly agreed to a longstanding request for the interview — made repeatedly after the first American seizure of Iranians here on Dec. 21 — and seemed eager to rebut the accusations.

The political and diplomatic standoff that followed the Dec. 21 raid until the Iranians were released nine days later has contributed, along with a dispute over the Iranian nuclear program, to greatly increased tensions between the United States and Iran. This month, American forces detained five more Iranians in a raid on a diplomatic office in the northern city of Erbil.

While providing few details, the United States has said that evidence gleaned in the Baghdad raid, made on an Iraqi Shiite leader’s residential compound, proves the Iranians were involved in planning attacks.

How much direction, if any, Mr. Qumi was taking from his government was unclear in the interview, in which he showed disdain for the American accusations as well as a few flashes of restrained sarcasm.

He ridiculed the evidence that the American military has said it collected, including maps of Baghdad delineating Sunni, Shiite and mixed neighborhoods — the kind of maps, American officials have said, that would be useful for militias engaged in ethnic slaughter. Mr. Qumi said the maps were so common and easily obtainable that they proved nothing.

He declined to say whether he believed the maps bore sectarian markings or address other pieces of evidence the Americans said they had found, like manifests of weapons and material relating to the technology of sophisticated roadside bombs. But that is not why the Iranians were in the compound, he said.

“They worked in the security sector in the Islamic Republic, that’s clear,” Mr. Qumi said, referring to Iran. But he said that the Iranians were in Iraq because “the two countries agreed to solve the security problems.” The Iranians “went to meet with the Iraqi side,” he said.

In a surprise announcement, Mr. Qumi said Iran would soon open a national bank in Iraq, in effect creating a new Iranian financial institution right under the Americans’ noses. A senior Iraqi banking official, Hussein al-Uzri, confirmed that Iran had received a license to open the bank, which he said would apparently be the first “wholly owned subsidiary bank” of a foreign country in Iraq.

“This will enhance trade between the two countries,” Mr. Uzri said.

Mr. Qumi said the bank was just the first of what he said would be several in Iraq — an agricultural bank and three private banks also intend to open branches. Other elements of new economic cooperation, he said, include plans for Iranian shipments of kerosene and electricity to Iraq and a new agricultural cooperative involving both countries.

He would not provide specifics on Iran’s offer of military assistance to Iraq, but said it included increased border patrols and a proposed new “joint security committee.”

Any Iranian military assistance to Iraq would be fraught with potential difficulties. Aside from provoking American objections, such assistance could further alienate Sunni Arabs, many of whom already suspect that Iran, overwhelmingly Shiite, is encouraging Iraq’s Shiite-led government in persecuting them.

A number of American and Iraqi officials said Sunday that it was difficult to respond to Mr. Qumi’s statements until they had been communicated through official routes. A spokesman for the American Embassy in Baghdad, Lou Fintor, declined to address the statements.

Sean McCormack, a State Department spokesman, said Sunday that the United States had a significant body of evidence tying Iran to sectarian attacks inside Iraq. “There is a high degree of confidence in the information that we already have, and we are constantly accumulating more,” Mr. McCormack said.

He did not address any of the specifics of Mr. Qumi’s comments about plans for stronger economic and security ties, but said that Iran currently plays “a negative role in many respects” in the country.

Iraqi officials also said that they could not comment on specific programs until they had seen the details, but expressed a range of views on the wisdom of expanding ties with Iran.

“We are welcoming all the initiatives to participate in the process of reconstruction,” said Qasim Daoud, a former national security adviser who is now a secular Shiite member of Parliament. “My belief is that our strategic alliance is with the Americans, but at the same time we are looking for the participation of any country that would like to participate,” Mr. Daoud said.

Barham Salih, a deputy prime minister who is Kurdish and whose duties include economic matters, took sharper issue with Mr. Qumi’s criticism of the American presence.

“Iraqi national interest requires seeking good neighborly relations with Iran as with other neighbors, but that requires respect for Iraqi sovereignty,” Mr. Salih said.

Mr. Qumi spoke largely in Persian during the interview, but he occasionally broke into English when he wanted to be certain that a point had been conveyed forcefully.

Although Mr. Qumi was not given specific questions before the interview, he was made aware of the general topics that would be covered and seemed prepared with detailed answers in many cases. He seemed keen to give his government’s view of what occurred in the early morning of Dec. 21, when American forces raided the Baghdad compound of Abdul Aziz al-Hakim, one of Iraq’s most powerful Shiite leaders, who had traveled to Washington three weeks before to meet President Bush.

Within the compound, the Iranians were seized in the house of Hadi al-Ameri, who holds two powerful positions: he is chairman of the Iraqi Parliament’s security committee and leader of the Badr Organization, the armed wing of Mr. Hakim’s party, which spent years in exile in Iran.

Although the Americans have suggested that the Iranians were providing support for militias like the Badr Organization, Mr. Qumi said that his countrymen were dealing with Mr. Ameri in his government capacity.

The Iranians would not even have stayed the night in the compound except, in a situation faced by many Baghdad residents, their business lasted beyond the early-evening curfew and they were forced to spend the night, Mr. Qumi said.

Mr. Qumi also warned the United States against playing out tensions in what he called “the nuclear file” in Iraq. “We don’t need Iraq to pay the cost of our animosity with the Americans,” Mr. Qumi said.

As the interview was breaking up, Mr. Qumi made one last stab at the Americans. If Iran is allowed to undertake reconstruction activities in Iraq, he said, all international construction companies would be welcome. “Urge the American companies to come here,” he said.