Head of Shin Bet says three attempted attacks by Iran have been thwarted in the past year

Iranian agents are attempting to attack Israeli targets around the world in retaliation for covert operations, including the assassination of Iranian nuclear scientists, the head of Shin Bet, the Israeli security agency, has warned.

"It doesn't matter if it's true or not that Israel took out the nuclear scientists. A major, serious country like Iran cannot let this go on. They want to deter Israel and extract a price so that decision-makers in Israel think twice before they order an attack on an Iranian scientist," Yoram Cohen said in a lecture reported in Haaretz.

Three attempted attacks were thwarted at the last minute in the past year, in Turkey, Azerbaijan and Thailand, he said.

Four Iranian nuclear scientists have been assassinated in the past two years in what Iran – and many others in the international community – believe are operations by the Israeli secret services, or its proxy agents, as part of a covert war.

Israel usually maintains a code of silence on activities by its security agencies, Shin Bet and the Mossad. But the president, Shimon Peres, said "to the best of my knowledge" the country was not involved in the most recent assassination, less than a month ago.

Cohen's warning of retaliatory attacks came amid mounting speculation Israel is moving closer to launching a unilateral military strike against Iran.

The US defence secretary, Leon Panetta, declined to deny claims in a column in the Washington Post that said he believed Israel would launch an attack in April, May or June this year. And Israel's defence secretary, Ehud Barak, told a security conference that the window for action would close when Iran reached an "immunity zone" with its enrichment activities moved deep underground beyond the reach of air bombardment. "Those who say 'later' may find that later is too late," he said.

Britain's deputy prime minister, Nick Clegg, this week voiced fears Israel could take unilateral military action against Iran. "Of course I worry that there will be a military conflict and that certain countries might seek to take matters into their own hands," he told the House magazine.

The Shin Bet chief also said Iran was building closer ties with Islamic Jihad in Gaza following its rift with Hamas over the Syrian uprising.

However, relations between Hamas and Tehran may be repaired when Ismail Haniyeh, the de facto prime minister of Gaza, visits the Iranian capital in the coming days.

During his invitation-only lecture in Tel Aviv on Thursday, Cohen said the security agency was concerned about extremist religious settlers in the West Bank who "have decided to take the road of terror … [lashing] out at Arabs and [their] sacred symbols".

This week, several rightwing activists from West Bank settlements were elected to the central committee of the Likud party, which is led by the prime minister, Binyamin Netanyahu. One, Meir Bartler, 27, considered a leader of the "hilltop youth", is under house arrest, according to the Israeli daily Ma'ariv.