The hope is that Iran, through its negotiator, Saeed Jalili, will agree to the offer as an agenda for rapid and continual negotiations that can convince the world that its nuclear program is purely civilian, as Iran insists.

Iran presented its own five-point package of proposals, which Iranian news reports described as containing both nuclear and non-nuclear elements. By late Wednesday, it was unclear whether any progress had been made. But the six powers — the United States, France, Britain, Russia, China and Germany — rejected Iranian calls for an immediate easing of economic sanctions imposed on Tehran for flouting United Nations Security Council resolutions demanding the suspension of all enrichment. Senior diplomats also said that harsher American and European Union sanctions on oil exports and banking transactions to go into effect in July would not be postponed.

The six want to buy more time for more comprehensive and detailed negotiations with Iran on the nature of its nuclear program, and so want to put a cap on what they view as the most urgent and sensitive issue, which is Iran’s growing stockpile of uranium enriched to 20 percent. Iran says the uranium is for fuel for medical reactors, but Western diplomats say Tehran already has many times more than it needs, and that moving from 20 percent to bomb-grade purity is a relatively quick and easy process.

The six powers also want Iran to export its current stockpile of 20 percent uranium and down the road, to dismantle the once-secret Fordo enrichment plant, deep inside a mountain near the holy city of Qum, that is producing it.

Here in Baghdad, Iran’s choice for a venue, the six powers presented an agenda for near-term action, Western diplomats said, designed to defuse the crisis by removing the 20 percent uranium that most worries Israel and other American allies like Saudi Arabia and the Gulf states. Israel has warned that it might attack Iran militarily if Tehran appears to be nearing completion of a nuclear weapon or it continues to produce highly enriched uranium in protected sites like Fordo, that are difficult to bomb.

“There’s a new offer on the table which address our concerns, not the least the 20 percent enriched uranium,” said Michael Mann, spokesman for Catherine Ashton, the European Union’s top foreign-policy official who lead the six-power negotiation team. The negotiations are a process, Mr. Mann said, and “these things can’t be resolved overnight.” If the talks go well here, he said, “we are going to make solid progress.”

In return for early Iran steps to freeze 20 percent, the six are offering benefits like spare parts for civilian aircraft, much needed in Iran, and help with nuclear safety at civilian installations, and perhaps a pledge that Iran has the right to a peaceful nuclear program so long as it clears up doubts about its intentions through serious, detailed, technical negotiations with the six and through openness with the inspectors of the International Atomic Energy Agency.

The six are also offering a new version of a fuel-swap program, to take Iran’s 20 percent enriched uranium and return it as processed fuel for medical reactors.

But there were indications that the Iranians want more in return, including the easing of more serious sanctions. Mr. Jalili is said to be citing increased cooperation with the I.A.E.A. as a reason for the six to ease sanctions.

The substance of the Iranian side’s proposal was not disclosed, but the Iranian media called it a “comprehensive proposal” with five elements, and Iranian journalists here said that it included a nuclear plank and one on regional issues.

“We need the steps that both sides have to take to be clearly defined and there is no possibility of going back on them,” an Iranian official was quoted as telling Agence France-Presse. If Western nationsy lift sanctions, for instance, he said, “they cannot then readopt them two months later under a different pretext.”

In Iran, the main 9 p.m. state television news broadcast devoted nearly 10 minutes to the negotiations in Baghdad, stressing that the package offered verbally by P5+1 was “unbalanced,” while lauding Iran’s own proposal.

The delegations met for three hours, broke for lunch and conversation, then resumed discussions into Wednesday evening before adjourning. Talks were scheduled to resume Thursday morning, diplomats said.

On Tuesday, the director-general of the I.A.E.A., Yukiya Amano, said that he had reached something of a breakthrough with Iranian officials on the agency’s longstanding request for access to some of Iran’s military facilities and interviews with key officials to check Iran’s assertion that it is not working on a nuclear weapon.

Mr. Amano’s assertion suggested that Iran was seeking to set a positive tone for the nuclear talks and perhaps ease pressure from strict Western-led sanctions that are about to become even more severe. Western diplomats feared that Mr. Jalili would use Mr. Amano’s assertions of new cooperation on Tuesday as a reason to demand an easing of sanctions, which has been Iran’s main aim in these talks.

Iranian officials sought to dispel the notion that they were acting under pressure. At a news conference in Tehran on Wednesday, Foreign Minister Ali Akbar Salehi said that “policies of pressure and intimidation are futile,” urging Iran’s interlocutors “to adopt policies to show good will to solve this issue.” He said Iran hoped that “in a day or two we can bring good news” from the Baghdad talks, Reuters reported.

Not directly mentioning the nuclear talks, Iran’s supreme leader, Ayatollah Ali Khamenei, speaking at a military academy on Wednesday, said that the West’s power is in decline. “No matter their propaganda, they are being weakened and destroyed.” Iran believes that it is playing a successful longer game for influence in the Middle East and Afghanistan as Washington withdraws.

Ayatollah Khamenei is considered the main decision-maker on the nuclear issue, and Mr. Jalili has been appointed his personal representative. President Mahmoud Ahmadinejad has been banned from interfering in the negotiations, analysts say, in order to allow the Islamic Republic to speak with one voice during the talks.

“The Iranian nation is hopeful of the future,” Ayatollah Khamenei said. “And the horizon of the future is smiling towards the Iranians.”

**Reporting was contributed by Alan Cowell from Paris and Thomas Erdbrink from Tehran.