The United States called on Iran on Monday to take "urgent practical steps" to build confidence during nuclear talks with world powers, and the European Union said Tehran must suspend sensitive atomic activities.
Iran and the six powers resumed discussions in mid-April in Istanbul after a gap of more than a year - a chance to ease escalating tension and help to avert the threat of a new Middle East war.
The major powers - the United States, France, Britain, China, Russia and Germany - and Iran are to meet again on May 23 in Baghdad.
"We remain concerned by Iran's persistent failure to comply with its nonproliferation obligations," US envoy Robert Wood told an international nuclear conference in Vienna, attended also by Iran.
"We seek a sustained process that produces concrete results, and call on Iran to take urgent practical steps to build confidence and lead to compliance with all its international obligations," Wood added.
At the same meeting, the 27-nation European Union said in a joint statement that Iran "must suspend" its uranium enrichment activities - something Tehran has repeatedly refused to do.
The West says Iran's nuclear work is a cover for developing atomic bombs and wants verifiable assurances to the contrary from Tehran - for example, by accepting much more intrusive UN inspections and curbing its enrichment capacity.
Israel and the United States have not ruled out military action to prevent Iran from building nuclear weapons if diplomacy fails to resolve the long-running row.
Iran denies having a weapons agenda, saying it is enriching uranium solely for peaceful energy purposes, not for bombs.
The week before the broader political negotiations take place in Baghdad, the UN nuclear agency and Iran will hold a new round of discussions on May 14-15 in Vienna after two meetings earlier this year failed to make any headway.
The International Atomic Energy Agency (IAEA) wants Iran to address questions raised in a report it issued last November detailing what it said were suspected Iranian research and development activities relevant to making nuclear weapons.
Iran has dismissed the allegations as fabricated.
Western diplomats say Iran appears to be stonewalling an IAEA request for access to a military site, Parchin, where it believes military-related nuclear research may have taken place.
The diplomats say they suspect Iran may be "sanitizing" the site southeast of Tehran of any incriminating evidence before UN inspectors can visit, a suspicion Tehran dismisses.
Wood said: "We are concerned that Iran has not agreed to grant the IAEA access to all relevant sites, information, documents and persons necessary to resolve questions about its nuclear program."
"We stress the urgent need for Iran to reach agreement with the IAEA on a structured approach ... to resolve all outstanding issues," Wood, acting head of the US mission to the Vienna-based IAEA, added.