"We're in close consultation with the UK, France and Germany, as well as other friends and allies, on Iran's nuclear program and the IAEA investigation," deputy State Department spokesman Adam Ereli said.
Britain, France and Germany have sent their foreign ministers to Teheran.
Ereli said the four countries were in agreement on the need for Iran to comply fully with the International Atomic Energy Agency (IAEA) verification requirements to show that Iran's nuclear program is not building weapons.
"We all support full compliance with the IAEA and Non-Proliferation Treaty (NPT) requirements without delay or conditions and we are confident that the foreign ministers will make this clear to Iran -- if they travel there -- and the need for such compliance."
Ereli was asked about the possibility that Iran might be offered compensation.
"It is our view that Iran's obligations under the NPT and the IAEA safeguards agreement are non-negotiable," he said.
German Foreign Minister Joschka Fischer arrived in Tehran late Monday, to be followed by his colleagues Jack Straw of Britain and Dominique de Villepin of France.
The European mission coincides with the official Iran news agency IRNA announcement that conditions were in place for Iran to sign an additional protocol to the NPT. That protocol would allow a stepped-up inspection regime at nuclear sites.