MOSCOW — Diplomats from six nations and Iran arrived in Moscow ahead of a new round of nuclear talks Monday that some American diplomats consider pivotal to determining whether or not to pursue the current incremental approach when it comes to negotiations over Iran's nuclear program.

“Tomorrow’s E3+3 talks in Moscow are important meetings,” a senior Western official told journalists in Moscow Sunday. ”Iran should come prepared to negotiate seriously and take concrete steps to address the unified proposal laid out” in Baghdad, where the last round of talks were held in May. ”Iran has a right to peaceful nuclear energy under the [Non-Proliferation Treaty], but it must first meet its international obligations... If Iran remains unwilling to take the opportunities these talks present, it will face continuing and intensified pressure and isolation.

Western diplomats said Sunday they have asked Iran to respond to a detailed proposal put forward by the six-nation negotiating group at the meeting last month in Baghdad. That package called for Iran to halt its production of 20 percent enriched uranium, ship out its 20 percent stockpile, and stop operations at Fordow, a highly fortified enrichment facility built into a mountain near Qom. In return, Iran would get spare parts for old US civilian airliners and fuel and safety upgrades for a reactor that makes medical isotopes.

But some American advisers say if Iran does not show a willingness in Moscow to seriously engage on that interim proposal, then the US should consider pivoting to a different, higher stakes, approach that they say would clarify Iran’s intentions.