"I have not taken a decision" on any attack against Iranian nuclear facilities, Netanyahu said in response to a question during an interview with the private television Channel 2.

But the prime minister reaffirmed "the right of Israel to defend against any threat to its security and existence."

"Israel's fate depends solely on us and no other country, however friendly," he said, in reference to the United States.

Asked about reports that the army, the Mossad intelligence agency, and the Shin Bet internal security service were against any attack launched without US consent, he said: "In a democracy, only the political leaders decide, and the military executes."

He recalled that then prime minister Menachem Begin had given the green light in 1981 for an air strike against a nuclear plant in Iraq "despite the opposition at the time of the chiefs of the Mossad and military intelligence."

"Political leaders have a more global vision and it is they who bear supreme responsibility," Netanyahu said in another interview with private television Channel 10.

His remarks came shortly after the arrival in Israel of US Defence Secretary Leon Panetta.

The Pentagon chief told a news conference in Cairo before flying to Jerusalem that he would share information with Israel on Iran's nuclear programme, although he said there would be no discussion about "potential attack plans."

"We have in the past and we'll continue to discuss the situation with regards to Iran and the threat that it poses in the region," Panetta told a news conference in Cairo before flying to Jerusalem.

"I think it's the wrong characterisation to say that we're going to be discussing potential attack plans. What we are discussing are various contingencies on how we would respond," said Panetta.

In a statement released by the White House, Obama said the new measures underlined the United States' determination to force Tehran "to meet its international obligations" in nuclear negotiations.

The sanctions came on the same day as the US State Department branded Iran "an active state sponsor of terrorism" in its 2011 annual terrorism report, and as US lawmakers prepared to vote legislation demanding more action.

Obama is keen to show his Iranian sanctions regime is tough, amid fears Israel may launch unilateral strikes against Iran if it believes the Islamic regime is on the point of achieving the capability to build a nuclear bomb.

"This action is designed to deter Iran from establishing payment mechanisms for the purchase of Iranian oil to circumvent existing sanctions," Obama said, warning that US sanctions will be apply to any entity buying Iranian oil.

Obama said measures would be taken against firms that have dealings with the National Iranian Oil Company, the Naftiran Intertrade Company or the Central Bank of Iran or that help Iran buy US dollars or precious metals.

And he accused the Bank of Kunlun in China and the Elaf Islamic Bank in Iraq of arranging transactions worth millions of dollars with Iranian banks already under sanctions because of alleged links to Tehran's weapons program.

Obama said these two institutions would henceforth be denied access to the US financial system, as would any banks caught dealing with Iran in future.

"Today's action makes it clear that we will expose any financial institution ... that allows the increasingly desperate Iranian regime to retain access to the international financial system," he said.

Separately, the US Treasury Department confirmed details of the sanctions and allegations against the Chinese and Iraqi banks, accused of dealing with Iranian firms accused of links to weapons proliferation and terrorism.

"As financial institutions around the world have cut ties with these designated Iranian banks, Bank of Kunlun and Elaf Islamic Bank took the opposite approach," the statement said.

The Treasury said the Beijing-based Bank of Kunlun provided services to at least six Iranian banks that have been placed under US sanctions because of their alleged roles in Iran's weapons of mass destruction programs.

In particular, it is alleged Kunlun made roughly $100 million in payments from accounts it held for the Iran's Bank Tejarat and made a payment on behalf of an entity linked to Iran's Islamic Revolutionary Guard Corps.

The Baghdad-based Elaf Islamic Bank, meanwhile, is alleged to have "engaged in activity during the past year worth tens of millions of dollars with the Export Development Bank of Iran."

Israel has its own undeclared nuclear arsenal, but fears a nuclear-armed Iran would shift the balance of power in an already volatile Middle East or that its fierce foe Tehran might try to eradicate the Jewish state.

During a visit to Tunisia on Monday at the start of a Middle East tour, US Defense Secretary Leon Panetta said sanctions were having a "serious impact" on the Iranian economy, even if their results may not be immediately obvious.