WASHINGTON - As nuclear negotiations between Iran and the West continue to move slowly, U.S. President Barack Obama is coming under growing pressure from what appears to be a concerted lobbying and media campaign urging him to act more aggressively to stop Iran's nuclear program.

Obama has given Tehran an end-of-September deadline to respond substantively to his offer of diplomatic engagement. But already hawks in the U.S. – backed by hardline pro-Israel organizations – have pressed him to quickly impose "crippling" economic sanctions against Tehran, and some are arguing that he should make preparations for a military attack on Iranian nuclear facilities.

The pressure campaign kicked off in earnest this week. On Thursday, hundreds of leaders and activists from the U.S. Jewish community descended on Washington to lobby for harsher sanctions, while widely-publicised media reports suggested that Iran is already nearing the verge of a nuclear capability.

Leaders from Jewish groups came for a national "Advocacy Day on Iran", during which they met with key Congressional figures.

Rep. Howard Berman, a California Democrat who heads the House Foreign Affairs Committee, suggested that the clock "has almost run out" on Iran's nuclear program, and indicated that he would move ahead next month with a bill imposing sanctions on Iran's refined petroleum imports "absent some compelling evidence why I should do otherwise".

The bill, the Iran Refined Petroleum Sanctions Act (IRPSA), has for months been the top lobbying priority of hawkish pro-Israel lobbying groups led by the American Israel Public Affairs Committee (AIPAC). To their frustration, Berman has held up consideration of the bill for most of the past year

Not all U.S. Jewish groups are lining up behind the legislation, however.

Americans for Peace Now (APN), for instance, issued a statement arguing that "arbitrary deadlines are a mistake" and that "pursuing sanctions that target the Iranian people, rather than their leaders, is a morally and strategically perilous path that the Obama Administration must reject".

M.J. Rosenberg, a foreign policy analyst at Media Matters Action Network, suggested on the website TPMCafe that the advocacy day "marks the start of the fall push on Iran".

The advocacy group United Against Nuclear Iran (UANI) has launched an intensive television advertising campaign this month claiming that the U.S. "must isolate Iran economically to prevent Iran from developing a nuclear weapon".

UANI's two co-founders are now both high-ranking officials in the Obama administration – Dennis Ross, currently overseeing Iran policy at the National Security Council (NSC), and Richard Holbrooke, now the State Department special representative in charge of Afghanistan and Pakistan.

Also on Thursday, the New York Times published a front-page story claiming that U.S. intelligence agencies believe "that Iran has created enough nuclear fuel to make a rapid, if risky, sprint for a nuclear weapon", although the article did not provide an estimate of when Iran could have a nuclear capability.

The same day, the Wall Street Journal published an op-ed by former Senators Charles Robb and Daniel Coats and retired four-star Air Force General Chuck Wald. Claiming that Iran "will be able to manufacture enough highly enriched uranium for a nuclear weapon in 2010", the authors urged Obama "to begin preparations for the use of military options" against Iran.

However, official U.S. intelligence estimates provide a far slower timeline. In February, Director of National Intelligence (DNI) Dennis Blair told Congress that Iran would be unable to produce highly enriched uranium (HEU) until at least 2013, and stated that there is "no evidence" that Iran had even made a decision to produce HEU.

Iran insists that its nuclear program is intended solely for civilian purposes. In 2007, the U.S. intelligence community released a National Intelligence Estimate suggesting that Iran had halted its nuclear weapons program in 2003.

The campaign comes on the eve of a series of key international meetings in late September, including the annual opening of the U.N. General Assembly in New York and the Group of 20 (G20) Summit in Pittsburgh.

Iran and its nuclear program are expected to be a major topic for world leaders who will attend these meetings, and hawks in Washington and Jerusalem hope that Obama will use them to push for the imposition of far-reaching economic sanctions by the U.N. Security Council as soon as possible.

While Obama faces pressure to move quickly to sanctions, the government of Iranian President Mahmoud Ahmadinejad is still struggling at home to overcome challenges to its legitimacy resulting from the disputed presidential election in June. Many analysts suggest that Iran's government is currently in no position to respond coherently to U.S. engagement.

This week, Ahmadinejad's government finally issued a formal reply to proposals by the P5+1 powers - the U.S., China, Russia, Britain, France, and Germany - for talks on its nuclear program and related issues.

But the five-page-reply has been deemed too vague by Washington, with State Department spokesman P.J. Crowley dismissing it Thursday as "not really responsive" to U.S. concerns.

Other analysts suggested that the Iranian proposal was more promising than initial media reports would indicate.

"Iran's uncompromising stance and its cursory references to nuclear matters are most likely an opening bid, and not a red line," wrote National Iranian American Council (NIAC) president Trita Parsi in the Huffington Post.

He suggests that the proposal's language "may offer an opening to push strongly for transparency and acceptance of intrusive inspections and verification mechanisms".

The Obama administration, however, continues to hold out hope for the engagement strategy.

"We'll be looking to see how ready Iran is to actually engage, and we will be testing that willingness to engage in the next few weeks," Crowley said.

At the same time, Russian Foreign Minister Sergei Lavrov all but ruled out his country's cooperation with new sanctions against Tehran at the Security Council, and called instead for renewed negotiations based on Iran's reply.

Lavrov's comments came shortly after a secret and still-mysterious visit to Russia by Israel's right-wing prime minister, Benjamin Netanyahu.

The latest developments - along with growing amount of attention being paid to U.S. policy in Afghanistan, at the expense of Iran - have only added to the frustration of Iran hawks in Washington. They believe increasingly that economic sanctions alone, even if they are imposed multilaterally, are unlikely to be enough to persuade Tehran to halt what they see as its drive to obtain a nuclear weapon.

For this reason, many suggest that the U.S. should either make preparations to attack Iran militarily itself, or step aside and allow Israel to do so.

"No one should believe that tighter sanctions will, in the foreseeable future, have any impact on Iran's nuclear weapons program," former U.N. ambassador John Bolton, a noted hardliner, wrote in the Wall Street Journal last month. "Adopting tougher economic sanctions is simply another detour away from hard decisions on whether to accept a nuclear Iran or support using force to prevent it."

Earlier that month, the Journal featured an article by Gen. Wald - who was one of the co-authors of Thursday's op-ed urging preparations for a military strike - entitled "Of Course There's a Military Option on Iran".

But critics suggest that the constant threats of military action against Tehran will only make the regime's leadership more intransigent on the nuclear issue.

"Pointing a gun at their heads merely reinforces their desire for a reliable deterrent, and probably strengthens the hand of any Iranian officials who think they ought to get a bomb as soon as possible," wrote Stephen Walt, a professor of international relations at Harvard University, on the website of Foreign Policy magazine.