Iran has postponed until 2014 the planned start-up of a research reactor which Western experts say could potentially offer the Islamic Republic a second route to produce material for a nuclear bomb, a UN report showed.

Tehran has continued to install cooling and moderator circuit piping in the heavy water plant near the town of Arak. Nuclear analysts say this type of reactor could yield plutonium for nuclear arms if the spent fuel is reprocessed, something Iran has said it has no intention of doing.

But the country has now delayed the planned timetable for bringing Arak on line by about half a year from the third quarter of 2013, according to the latest UN information in a confidential report submitted to member states late on Friday.

"Iran stated that the operation of the IR-40 reactor was now expected to commence in the first quarter of 2014," the International Atomic Energy Agency (IAEA) report said. It gave no reason for the postponement.

The Arms Control Association, a Washington-based research and advocacy group, said it was questionable whether Iran would be able to meet this new target date as well, in view of "significant delays and impeded access to necessary materials".

The West's worries about Iran are focused largely on underground uranium enrichment plants at Natanz and Fordow, but experts say Arak is also a possible proliferation concern.

Iran, rejecting Western allegations it seeks to develop a capability to assemble atomic arms, says its nuclear program is entirely peaceful and that the reactor will produce isotopes for medical and agricultural use.

Israel, believed to be the Middle East's only nuclear-armed state, sees Iran's nuclear program as a serious danger and has threatened to attack its atomic sites if diplomacy fails to resolve the decade-old dispute.

If it does, the nuclear sites at Natanz, Fordow and Arak in central Iran are likely to be among the targets.

Friday's quarterly IAEA report showed Iran pressing ahead with expanding its uranium enrichment program in defiance of tightening Western sanctions.

Enriched uranium can be used to fuel nuclear power plants, Iran's stated aim, but also provide the explosive core of a nuclear weapon if refined much further. Making plutonium from spent fuel is a second way of obtaining potential bomb material.

In August, German prosecutors said police had arrested four men suspected of delivering valves for the heavy-water reactor, breaking an embargo on such exports to Iran.

If operated optimally, the heavy-water plant would produce about 9 kilograms of plutonium annually, or enough for about two nuclear bombs each year, according to the Institute for Science and International Security, a U.S.-based think-tank.

"Before it could use any of the plutonium in a nuclear weapon, however, it would first have to separate the plutonium from the irradiated fuel," it added on its web site.

Iran has announced it has no plans to reprocess the spent fuel, the International Institute for Strategic Studies think-tank said in a report last year.

But Mark Fitzpatrick, director of its non-proliferation and disarmament program, has said that "similarly sized reactors ostensibly built for research" have been used by India, Israel, North Korea and Pakistan to make plutonium for weapons.