[caption id="attachment_111621" align="alignright" width="187"] Director General of the International Atomic Energy Agency (IAEA) Yukiya Amano[/caption]

(Reuters) - The U.N. nuclear watchdog chief said on Monday Iran was being slow to cooperate with his agency's investigation into the Iranian atomic program and that the inquiry could not continue indefinitely.

Diplomats have voiced doubt over whether the outstanding issues in the U.N. investigation would be resolved before a broader diplomatic agreement is reached between Iran and the United States, Britain, France, China, Russia and Germany.

The seven countries have set a deadline of late March for a framework deal and June for a comprehensive final settlement that would curb Iran's nuclear activity to ensure it cannot be put to bombmaking in return for the lifting of international sanctions that have hammered the oil-based Iranian economy.

When asked about a time frame for the U.N. inquiry running parallel to the higher-level negotiations, International Atomic Energy Agency chief Yukiya Amano said: "It depends on the level and pace of cooperation from Iran, I cannot tell by when...

"We have asked questions and the questions are clear, so (Iran) can answer."

The Islamic Republic has yet to address two outstanding issues relating to alleged explosives tests and other measures that might have been used for nuclear bomb research which it should have explained away by last August.

The West fears Iran wants to develop atomic bomb capability. Tehran says its program is for peaceful nuclear energy only.

The IAEA remains ready to accelerate the resolution of all outstanding issues, Amano added, but "this process cannot continue indefinitely". Iran's leading negotiator Abbas Araqchi met with Amano last week, promising swifter cooperation, but neither side has spelled out any details.

Iran wants Western countries to swiftly lift crippling economic sanctions in any deal curbing its nuclear program - one of the sticking points in high-level negotiations continuing in Switzerland this week.

The IAEA is likely to monitor the implementation of any deal between Iran and the six powers. Amano said he proposed a 1.8-percent increase to the body's 344-million-euro ($386 million) budget given increased demand for its services.

He reiterated deep concern about the nuclear activities of North Korea, which quit the nuclear Non-Proliferation Treaty in 1993. The IAEA has not had inspectors on the ground there since they were expelled by North Korea in 2009.