(Reuters) - Defence secretary Leon Panetta believes there is a growing possibility Israel will attack Iran as early as April to stop Tehran from allegedly building a nuclear bomb, US media have reported.

The Washington Post reported that Panetta was concerned about the increased likelihood that Israel would launch an attack over the next few months. CNN confirmed the report, citing an unidentified senior Obama administration official.

Panetta believes there is a strong likelihood that Israel will strike Iran in April, May or June - before Iran enters what Israelis described as a 'zone of immunity' to commence building a nuclear bomb, Washington Post columnist David Ignatius wrote.

Very soon, the Israelis fear, the Iranians will have stored enough enriched uranium in deep underground facilities to make a weapon - and only the United States could then stop them militarily, Ignatius wrote.

Panetta and the Pentagon declined to comment.

Israel, widely believed to possess the Middle East's only nuclear arsenal, views Iran's uranium enrichment projects as a major threat and has not ruled out the use of military force to prevent Tehran from acquiring nuclear weapons.

Iran says its nuclear programme is solely for peaceful purposes.

The Post article said the postponement of a joint US-Israeli military exercise that had been scheduled for the spring may have signalled the prospect of an Israeli attack soon.

Washington and the European Union have imposed tighter sanctions on Iran to force Tehran to provide more information on its nuclear programme.

Iran has said repeatedly it could close the vital Strait of Hormuz shipping lane if sanctions prevented it from exporting crude, a move Washington said it would not tolerate.

Israel's military intelligence chief said he estimated that Iran could make four atomic bombs by further enriching uranium it had already stockpiled, and could produce its first bomb within a year.

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

Israeli defence minister Ehud Barak said: If sanctions don't achieve the desired goal of stopping [Iran's] military nuclear programme, there will be a need to consider taking action.