The United States has powerful bombs at the ready in the case of possible military action against Iran and work is under way to bolster their firepower, the air force chief said Wednesday.

General Norton Schwartz, air force chief of staff, declined to say whether US weapons -- including a 30,000-pound massive ordnance penetrator (MOP) bomb -- could reach nuclear sites in Iran that were concealed or buried deep underground.

"We have an operational capability and you wouldn't want to be there when we used it," said Schwartz, when asked about the MOP bomb.

"Not to say that we can't continue to make improvements and we are," he told defense reporters.

Amid speculation that a nuclear site dug into the side of a mountain near Qom is beyond the reach of American weapons, Defense Secretary Leon Panetta has acknowledged shortcomings with the giant MOP bomb and said the Pentagon was working to improve the explosive.

"The bottom line is we have a capability but we're not sitting on our hands, we'll continue to improve it over time," Schwartz said.

Asked about recent comments from retired senior officers that some targets in Iran are immune from US air power, Schwartz said: "It goes without saying that strike is about physics. The deeper you go the harder it gets."

But he added that the US arsenal "is not an inconsequential capability."

The former vice chairman of the Joint Chiefs of Staff, retired general James Cartwright, suggested last week that one nuclear facility in Iran could not be taken out in a bombing campaign.

Cartwright appeared to be referring to the Fordo plant built deep inside a mountain near the Shiite shrine city of Qom, some 150 kilometers (90 miles) south of Tehran.

Schwartz also declined to say whether air power would be effective against Iran's nuclear program but said that the outcome of any preemptive attack would depend on the goal of the strike.

"What is the objective? Is it to eliminate, is to delay, is to complicate? I mean what is the national security objective. That is sort of the imminent argument on all of this," he said.

"There's a tendency I think for all of us to get tactical too quickly and worry about weaponeering and things of that nature."

The general's carefully calibrated remarks coincided with a visit to Washington this week by Israel's defense minister, Ehud Barak, amid renewed speculation of a potential Israeli strike on Iran's nuclear program.

Netanyahu takes Iran nuclear fears to Washington
Jerusalem (AFP) Feb 29, 2012 - Israeli premier Benjamin Netanyahu will carry his warnings on the dangers of a nuclear Iran to the White House next week for talks amid renewed speculation over an Israeli attack on the Islamic Republic.

Netanyahu has already made clear that Iran's nuclear programme, which Israel fears masks a weapons drive, will top his agenda in Washington.

"There is no doubt that one issue will be at the centre of our talks, and that is, of course, the continued strengthening of Iran and its nuclear programme," he told his cabinet on Sunday.

Experts in Israel say Netanyahu's discussions with President Barack Obama will be a chance for the allies to sound each other out on their sometimes divergent positions on Tehran's nuclear programme.

"The prime minister wants to make Iran the central subject and I think the president this time does as well," Israeli political scientist Jonathan Rynhold told AFP.

"There is a major disagreement between the (US) administration and the Israeli government about where the red line is on the Iranian nuclear programme," said Rynhold, of Bar Ilan University, near Tel Aviv. "That will be a major issue of tension."

Netanyahu and Obama are to meet on Monday and the Israeli premier will address a convention of the American Israel Public Affairs Committee lobby group that evening.

It will be his first visit to the United States since September, when he met the president on the fringes of the UN General Assembly.

Obama is to speak to AIPAC on Sunday, when Netanyahu arrives in Washington, after a weekend stopover in Canada, where he will meet Prime Minister Stephen Harper, a keen backer of Israel.

"It's basically a courtesy call and a thank you call in terms of Canada's outspoken support for Israel," Zachariah Kay, a scholar at Jerusalem's Hebrew University and founder member of the Israel Association for Canadian Studies, said of the Ottawa visit. "Keeping friends intact is the important point."

Some of its other friends have been piling pressure on Israel to desist from attacking Iran and allow time for a regime of international sanctions to kick in.

US military chief Martin Dempsey, in an interview with CNN earlier this month, warned that it would be "premature" to launch military action against Iran.

"Israel will be under a lot of pressure to give these sanctions time to work," Rynhold said, adding that the United States could offer its own guarantees to the Jewish state.

"I think that if the administration were to give the Israeli government the impression that they would actually use force to prevent Iran going nuclear then that would definitely serve to tip the balance in the Israeli government in terms of waiting considerably longer."

"I think that in any event Israel's unlikely to do anything in the next three months because President Obama will make a very strong case that sanctions have to be given time and I think that argument is likely to win, at least in the short term."

Iran has topped the agenda in US-Israel talks of recent weeks, which have included visits here by US National Security Adviser Tom Donilon and US intelligence chief James Clapper.

Israeli Defence Minister Ehud Barak left for Washington on Monday on a two-day US trip expected to include meetings with Vice President Joe Biden, Defence Secretary Leon Panetta, and Donilon.

The United States is not alone in wanting to rein in Israel, the sole if undeclared nuclear power in the Middle East.

British Foreign Secretary William Hague has said it would not be "wise" for Israel to take military action against Iran, echoing comments earlier this month by French President Nicolas Sarkozy.

In 1981, Israel launched a pre-emptive strike on the unfinished Osirak reactor outside Baghdad, leaving US officials stunned and earning it a sharp rebuke from its American ally.

For now, Israel says it is keeping all options open for dealing with Iran's nuclear programme, which much of the international community fears masks a weapons drive, despite Tehran's denials.

"The state of Israel is a sovereign state; it has the right and capacity to defend (itself) against any threat," President Shimon Peres said last week. "When we say that all options are on the table, we really mean it."