Iran aims to start a reactor next year which the West fears could arm an atomic bomb; Israel, which has bombed such construction sites around the Middle East before, may try to stop the plant being completed, say experts.

The timetable for the planned start-up of the Arak heavy-water research plant is closely watched: Israeli and Western experts say any attacker would probably prefer to act before it becomes operational - to avoid generating radioactive fallout.

The Islamic Republic says it will make isotopes for medical and agricultural use. But analysts say this type of facility can also produce plutonium for weapons if the spent fuel is reprocessed - something Iran says it has no intention of doing.

Time may be pressing for adversaries who want to act.

"Whoever considers attacking an active reactor is willing to invite another Chernobyl," former Israeli military intelligence chief Amos Yadlin said, referring to the 1986 Soviet reactor accident which sent radioactive dust across much of Europe.

"And there is no one who wants to do that," the experts added.

Yadlin, who runs Tel Aviv University's Institute for National Security Studies, was among eight pilots who in 1981 bombed Iraq's Osirak reactor. Still under construction, Israel believed nuclear fuel was about to be loaded and decided to hit it then, avoiding a risk of sending fallout over nearby Baghdad.

Further underlining Israel's determination to prevent its enemies from acquiring the means to assemble nuclear bombs, it attacked a site in Syria in 2007 that the United States said was a reactor being built with North Korean help. Syria denied that.

Israel, widely assumed to be the Middle East's only nuclear-armed state, now sees Iran's purportedly civil nuclear program as the most serious risk to it and has threatened military action if diplomacy and sanctions fail to make Tehran hold back.

Western and Israeli worries about Iran are focused largely on uranium enrichment plants buried underground at Natanz and Fordow, as such material refined to a high level can provide the fissile core of an atomic bomb.

But diplomats and experts say Arak could offer Iran a second route to nuclear bombs, if it decided to build such arms.

"The concern about that plutonium route and the Arak site has got much stronger," said a Western diplomat in Vienna, where the UN nuclear agency is based. "I think it is another red line," added the envoy, who is not from one of the big powers.

It was a reference to a "red line" for Iran set by Israeli Prime Minister Benjamin Netanyahu in September, when he told the United Nations that Tehran must not be allowed to amass an amount of medium-enriched uranium that could, if refined further in a relatively uncomplicated process, make even a single bomb.

Iran has since kept its medium-enriched uranium stockpile below that point by converting some of it to make reactor fuel, in effect postponing any deadline for Israeli military action.

Israeli Strategic Affairs and Intelligence Minister Yuval Steinitz said Arak was "definitely a concern" but Iran's uranium enrichment track was still a "far more pressing" issue.

Iran says its nuclear work is a bid to generate electricity and also to make progress in other areas of scientific research. It denies accusations it is seeking to develop atomic weapons.

Mark Fitzpatrick, director of the non-proliferation and disarmament program of the International Institute for Strategic Studies (IISS), said Arak would potentially be able to produce one bomb's worth of weapons-grade plutonium a year.

It would thus, he said, be "one of the key targets if there is ever a decision by Iran's adversaries to employ military force against the nuclear program".

But Cliff Kupchan, Middle East analyst at consultancy Eurasia Group, said it would be politically difficult to strike a site that is monitored by inspectors from the International Atomic Energy Agency (IAEA), the UN nuclear agency.

"My instinct is that ... Israeli triggers would relate more directly to intelligence that Iran is building or seeks" the capability to reprocess atomic fuel, Kupchan said.

Iran says it does not have any reprocessing activities - which would be required to obtain plutonium from reactor fuel.

It already has one reactor, at the Bushehr power station on the Gulf, but Western governments do not see that as a military risk since it is of a type less suited to generating plutonium; Russia helped build the plant, begun under the U.S.-backed Shah in the 1970s, and also insists on taking back the spent fuel.

Tehran argues that it is Israel's nuclear arsenal - believed to have been assembled from plutonium produced at the secretive Dimona reactor - that threatens regional peace and stability.

But Iran's refusal to suspend nuclear activity with both civilian and potential military applications in defiance of UN Security Council demands and its lack of full openness with the IAEA have fuelled suspicions abroad about its ultimate aims.

An IAEA report issued to member states on May 22 showed Iran pressing ahead with the construction of Arak, including the delivery to the site of the reactor vessel.

Stressing it was urgent that Tehran provided it with design information about the plant, the U.N. agency said Iran planned to commission the reactor with nuclear fuel in the first quarter of 2014 and launch it in the third quarter of next year.

"It is quite an ambitious timetable," one diplomat said.

Fitzpatrick said he did not expect the reactor - located southwest of Tehran - to become operational until 2015.

Many experts have voiced doubt that Israel's conventional forces would be able to deliver lasting damage to Iran's distant, dispersed and fortified nuclear facilities.

But, asked if the above-ground Arak site would be an easier target than Fordow and Natanz, former Israeli air force chief Ido Nehushtan indicated that it may be:

"Obviously," he said, "There is a big difference in terms of the quality and quantity of targets."-Reuters