Iran's envoy in Vienna described ongoing discussions with International Atomic Energy Agency officials as "good" on Tuesday.
Before entering the second day of talks, Ali Asghar Soltanieh told reporters the atmosphere of the talks had been "very constructive."
On Monday, the nuclear watchdog again urged Iran to give it access to the sites, people and documents it seeks in accordance with Tehran's obligations under the Nuclear Non-Proliferation Treaty.
The IAEA, United States, Israel, many European nations, and Gulf Arab states believe Tehran is trying to develop nuclear weapons.
Iran maintains its nuclear program is for strictly peaceful purposes, but continues to deny IAEA inspectors access to its Fordow and Natantz enrichment sites, and the Parchin military site near Tehran.
Officials suspect Iran has built a chamber there intended for specialized high explosives tests, necessary for detonating fissile material.
Western diplomats charge Tehran of trying to remove incriminating evidence before allowing UN inspectors inside the facility - a charge not without precedence. Previous IAEA reports at other sites have indicated evidence of cover ups.
Iran has dismissed the allegations as being "childish" and "ridiculous."
Of additional concern is that Iran's drive to enrich its uranium stockpiles to 20% purity far outstrips the 3% necessary for power production, and is occurring in higher quantities than would be necessary for researching medical isotopes.
An IAEA report issued in November said the nuclear watchdog believed Tehran had conducted nuclear tests of a military nature, prompting its director Yukiya Amano to charge "Iran is not telling us everything."
The P5+1 - the five permanent members of the UN Security Council plus Germany - are intently watching the Vienna meetings ahead of a second round of talks with Iranian official in Baghdad later in the month.
The first round of talks, held in Istanbul in early April, only produced a resolution for more talks and led Israeli officials to charge Iran is using negotiations to stall for more time in its bid for nuclear weapons.
Israel, which has said it will use military force to stop Iran from obtaining nuclear weapons if necessary, is widely believed to be preparing a strike on Iran's nuclear sites.