WASHINGTON (AFP) - The White House warned United States (US) lawmakers on Tuesday that tightening sanctions on Iran could box America into a "march to war" and derail a diplomatic push to limit Tehran's atomic programme.

The warning marked a significant toughening of President Barack Obama's stance towards skeptical US lawmakers as he prepares to resume high-stakes nuclear diplomacy with Iran later this month.

"The American people do not want a march to war," White House spokesman Jay Carney told reporters.

Mr Obama has vowed he will not allow Iran to develop a nuclear weapon, but last week intense negotiations between Iran and six world powers failed to reach an interim deal to halt its programme.

This setback fuelled scepticism in Congress about the administration's plans to freeze planned new economic sanctions.

Secretary of State John Kerry heads to Capitol Hill on Wednesday to make the case for continued diplomacy.

Meanwhile, the White House prepared the ground by warning that limiting Mr Obama's scope to negotiate could leave him little option but a recourse to military force against Tehran's nuclear operations.

Key senators, some responding to Israel's denunciation of the proposed agreement, are framing plans to stiffen sanctions or to curtail Mr Obama's power to ease current measures.

But the White House implicitly warned that new sanctions could embolden hardliners in Tehran who oppose talks, and force Mr Obama to begin preparations for military action.

Mr Carney said Americans "justifiably and understandably prefer a peaceful solution that prevents Iran from obtaining a nuclear weapon, and this agreement, if it's achieved, has the potential to do that.

"The alternative is military action," Mr Carney warned.

"It is important to understand that if pursuing a resolution diplomatically is disallowed or ruled out, what options then do we and our allies have to prevent Iran from acquiring a nuclear weapon?"

White House aides privately say that once war-weary Americans understand the alternative to a deal with Iran means another Middle East conflict, they will will warm to Mr Obama's approach.

Officials have also warned a hardline stance by Congress would strengthen hardliners in Iran opposed to dialogue between new Iranian President Hassan Rohani's envoys and Washington.

Mr Kerry will take the administration's position directly to the Senate Banking Committee, which is mulling a new sanctions package.

"The secretary will be clear that putting new sanctions in place would be a mistake," State Department spokesman Jen Psaki said.

"What we are asking for right now is a pause, a temporary pause in sanctions," she told reporters. "We are not rolling them back."

The US and other western nations believe Iran is covertly trying to develop nuclear weapons, a charge Tehran denies.

The House of Representatives has already passed a bill hardening up the sanctions, but the Senate agreed to delay further action to allow diplomacy a chance to succeed.

Both Republicans and Democrats have grown increasingly sceptical.

Senate Foreign Relations Committee chairman Robert Menendez said in a USA Today op-ed that new sanctions are a "necessary insurance policy" to ensure Iran negotiates in good faith.

"We cannot substitute wild-eyed hope for clear-eyed pragmatism given Iran's record of deception," he said.

And he branded it "incompatible" for Tehran to be pursue talks while installing centrifuges and developing a heavy-water reactor.

"Tougher sanctions will serve as an incentive for Iran to verifiably dismantle its nuclear weapons programme. When Iran complies, sanctions can be unwound and economic relief will follow," he said.

An aide to Banking Committee chairman Tim Johnson said the senator "will not make a decision on additional sanctions until he has had a chance to consult with his colleagues following the briefing" by Mr Kerry on Wednesday.

The committee's top Republican, Senator Mike Crapo, has said he wants to "move ahead expeditiously" with a new sanctions regime.

"I don't see how we should adjust our sanctions policy before there is any progress on the negotiation," he told Politico last week.

Mr Colin Kahl, director of the Middle East Security Program at the Center for a New American Security, said Congress was warming to the idea that sanctions pressure got Iran to the negotiating table and "more pressure will get them over the goal line." But Mr Kahl told reporters that "Congress should be mindful... of doing things that would arm hardliners with the argument that the West isn't serious" about engaging Iran diplomatically over its nuclear programme.

Mr Trita Parsi, president of the National Iranian American Council, agreed, saying new sanctions could seriously limit Mr Rohani's ability to maintain his "soft position" on the negotiations.

"The hardliners are waiting to destroy him," Mr Parsi said.

Negotiators "need to strike a deal before the Congress comes out and essentially closes the window of diplomacy".