Israeli Prime Minister Benjamin Netanyahu said on Monday that Iran was now closer to crossing the "red line" after which it would be able to build a nuclear weapon but had not yet reached that stage.

"The Iranians are closer to the red line that I set at the United Nations," his office quoted him as telling visiting American Jewish leaders. "They haven't crossed it yet but they are shortening the time needed to cross it."

"This must be stopped," he said. "We need to apply stronger pressure and harsher sanctions."

In a September address to the UN General Assembly, Netanyahu called for a "clear red line" to stop Iran getting a nuclear bomb.

He used a red marker pen to draw a line through a cartoon diagram of a bomb to illustrate what the international community's limit for Iran's uranium enrichment program should be.

He said Iran had 70 percent of the necessary level of uranium enrichment for a bomb and warned that at the current pace, the Islamic republic could have nearly all the material needed to create a first bomb by summer.

Netanyahu has publicly aired his differences with the United States over the Iran issue, criticising Washington for failing to set its own "red lines" that would trigger military action against Tehran.

President Barack Obama favours diplomacy and international sanctions against Iran to rein in its atomic programme.

The Iranian government says it is enriching uranium to 20 percent purity -- a short technical step from the 90 percent needed for a nuclear bomb -- for a medical research reactor. The West believes the effort hides a military goal.

Much of the international community fears Iran's nuclear programme includes efforts to develop nuclear weapons, a charge Tehran has repeatedly denied.

Israel, the Middle East's sole, albeit undeclared, nuclear power, believes Iran must be prevented from reaching military nuclear capabilities at any cost and refuses to rule out military intervention to that end.

Netanyahu last week also accused Iran of complicity in a bombing last year that killed five Israeli tourists at Bulgaria's Burgas airport.

The Bulgarian government said that the Lebanese Shiite militia Hezbollah was behind it.

"The attack in Burgas was just one in a series of terror attacks planned and carried out by Hezbollah and Iran," Netanyahu said on Tuesday. "That is in addition to the support that Hezbollah and Iran give to the murderous... regime in Syria.

Tehran has denied any involvement in the Burgas attack.