Israel estimated on Thursday that Iran could make four atomic bombs by further enriching uranium it has already stockpiled, and could produce its first within a year of deciding to build one.

But in his rare public remarks, Major-General Aviv Kochavi, chief of military intelligence, held out the possibility stronger international sanctions might dissuade Tehran from pursuing a policy he had no doubt was aimed at developing nuclear weapons, despite Iranian denials.

Citing figures similar to those from the U.N. nuclear agency, Kochavi told Israel's annual Herzliya Conference on strategic affairs: "Iran has accumulated more than 4 tonnes of uranium enriched to a level of 3.5 per cent and nearly 100 kilos at an enrichment level of 20 per cent.

"This amount of material is already enough for four atomic bombs."

Nuclear bombs require uranium enriched to 90 per cent, but Western experts say much of the effort required to get there is already achieved once it reaches 20-per cent purity, shortening the time needed for any nuclear weapons 'break-out'.

One former U.N. inspector said last month Iran could have enough 20-per cent uranium for one bomb - about 250 kg of the material - in about a year from now.

Tehran says it will use 20 per cent-enriched uranium to convert into fuel for a research reactor making isotopes to treat cancer patients. Western officials say they doubt that the country has the technical capability to do that.

Referring to Iranian Supreme Leader Ayatollah Ali Khamenei, in whose country's hands Israel believe a nuclear weapon would threaten the survival of the Jewish state, Kochavi said:

"From the moment Khamenei gives an order ... to speed up production of the first nuclear explosive device, we estimate it will take about a year to complete the task."

Arming a missile with a nuclear warhead, he added, could take a year or two longer.

Western experts' estimates of how quickly Iran could assemble a nuclear weapon if it decides to do so range from as little as six months to a year or more. Some believe Iran hopes to develop nuclear technology but stop short of building weapons, a move from which it is barred by treaty commitments.

In a report in November, the United Nations' International Atomic Energy Agency said Iran had almost 5 tonnes of low-enriched uranium and, citing data from September, 73.7 kg of uranium with a purity of 20 per cent.

'Strong data'

"Iran continues to contend that its program is for peaceful and civilian purposes," Kochavi said.

"But a long series of solid, strong data in our hands prove beyond any doubt that Iran is continuing to engage in developing nuclear weapons," he said in the speech, in which he steered clear of discussing Israel's military options.

Israel, widely believed to possess the Middle East's only nuclear arsenal, has said it would use force if necessary to prevent Tehran from acquiring nuclear weapons.

It has made little comment on Iranian accusations that its agents, along with those of its Western allies, are behind assassinations and explosions that appear to form part of a covert war to sabotage Iran's nuclear development capacity.

In separate remarks in Tel Aviv, Israeli Defense Minister Ehud Barak said 'we are in a period of diplomacy and sanctions' in trying to curb Iran's nuclear ambitions.

"It is clear to all that there is no need to take any option off the table, that there needs to be intensive and urgent diplomacy and that sanctions on Iran need to include not only on oil but on the financial system and the central bank," Barak told reporters.

Washington and the European Union have imposed tighter sanctions in recent weeks on both Iran's oil exports and on international financial transactions with Tehran.

Kochavi said the current sanctions have not led to a change in Iranian strategy, but could still have an effect.

"But the stronger the (pressure), the greater the potential for the regime - which is worried first and foremost about its survival - to reconsider," he said.

Tension between Iran and the West over Iran's nuclear work has increased since November, when the IAEA published a report that said Tehran appeared to have worked on designing a nuclear weapon.

Iran says its nuclear energy program is peaceful and aimed at generating electricity and other civilian uses.