Officials from the IAEA, the UN nuclear watchdog, were scheduled on Tuesday to wrap up a three-day visit to Iran seen as a chance to defuse an intensifying international showdown over Tehran's atomic activities.
But even as the high-stakes mission wound down, US lawmakers signalled they intended to keep up the pressure on the Islamic republic by unveiling plans for yet more economic sanctions, on top of those already infuriating Iran.
The visit came amid a building confrontation between Iran and the West, and speculation that Israel is planning military strikes on Iranian nuclear sites.
Iran's foreign minister, Ali Akbar Salehi, said on Monday his country was prepared to host the International Atomic Energy Agency officials for longer, "if they want" to extend their mission.
It was not known if the offer was made officially to the IAEA team, whose visit was taking place entirely out of public view.
The IAEA has kept silent about which Iranian officials the six-person team -- led by chief inspector Herman Nackaerts -- was talking with or if it was inspecting any suspect nuclear sites, and media in Tehran well being kept well away.
The UN agency has said the team was to focus on suspicions set out in a November 2011 report it issued strongly suggesting Iran was researching a nuclear weapon.
Iran has called that report baseless and maintains its nuclear programme is peaceful.
Its response to recent, severe Western economic sanctions against its finance and all-important oil sectors has been to defiantly ramp up its nuclear activities.
It has started uranium enrichment at a new fortified bunker in Fordo, near its holy city of Qom, and announced that a 20-percent enriched uranium fuel plate would be inserted into its Tehran research reactor within weeks.
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At the same time, though, it has vowed to keep up cooperation with the IAEA.
It has also voiced willingness to resume talks with world powers over its nuclear programme that collapsed a year ago, although it has yet to make any formal step in that direction.
Tehran's position, repeated by Salehi, is to call on the European Union and the United States to "replace their policy of sanctions with interaction" with the Islamic republic.
But key US lawmakers on Monday said a senate banking commission would soon vote a text to punish Iran further with more economic and political sanctions.
The legislation "sends a clear signal through strong measures that Iran must abandon its nuclear weapons program and its designs for the spread of international terror," said the top Republican senator on the panel, Richard Shelby.
The bill targets firms that have anything to do with helping Iran mine, produce or transport uranium anywhere in the world.
It also requires US-listed companies to disclose if they or their affiliates could have run afoul of US sanctions on Iran by investing in energy investments, or through the sale of communications monitoring or surveillance technology.
The bill would additionally deny US visas to Iranian students wanting to study in energy-related fields if it is deemed they plan to return to work in Iran's energy sector or nuclear programme.
And it would tighten sanctions on Iran's elite Revolutionary Guards, including targeting "anyone who materially assists" the Guards.