The United States has said it will continue to press Iran to fulfill its international obligations in the nuclear sphere after the UN nuclear watchdog had reiterated its suspicions over the nature of Tehran's nuclear program.

The International Atomic Energy Agency (IAEA) said in a report on Monday that Iran's lateness in admitting to a second uranium enrichment site raises concerns that it may be hiding further information on its nuclear activities.

"We will continue to press Iran in ways consistent with the dual-track approach to meet its international nuclear obligations," State Department spokesman Ian Kelly said in a statement on Monday.

Iran admitted to the existence of an enrichment site near the city of Qom in September, and IAEA inspectors, who were granted access to the facility the following month, said construction was at an advanced stage.

According to the report, Iran told the IAEA that it had begun building the facility in 2007, but inspectors believe that the project began in 2002, was paused in 2004, and resumed in 2006.

The United States and other Western nations fear Iran's nuclear program could be a cover for weapons production. Tehran says it needs nuclear technology to produce electricity.

"Now is the time for Iran to signal that it wants to be a responsible member of the international community," Kelly said.