The US Senate stood poised Thursday to adopt legislation imposing harsh new economic sanctions on Iran despite an 11th-hour plea from top aides to President Barack Obama to reject the measure.

Undersecretary of State for Political Affairs Wendy Sherman and Treasury Undersecretary for Terrorism and Financial Intelligence David Cohen warned the plan risked alienating key allies and inadvertently lining Iran's pockets.

"We all agree with the impulse, the sentiment, the objective, which is to really go at the jugular of Iran's economy," Sherman said in a frequently contentious hearing of the Senate Foreign Relations Committee.

"But there is absolutely a risk that, in fact, the price of oil would go up which would mean that Iran would, in fact, have more money to fuel its nuclear ambitions, not less," she said.

"Now, more than ever, it is imperative that we act in a way that does not threaten to fracture the international coalition of nations," Cohen said at a hearing just hours before lawmakers were to vote on the plan.

The measure, crafted by Democratic Senator Robert Menendez and Republican Senator Mark Kirk, aims to cut off Iran's central bank from the world financial system in a bid to force Tehran to freeze its suspect nuclear program.

Senators seeking new leverage against what the West charges is a covert atomic weapons quest were due to attach the plan to a must-pass annual military spending bill, and then approve the underlying legislation as early as Thursday.

The legislation would freeze the US-based assets of any financial institution that does business with the central bank, including foreign central banks that do so for the purposes for buying or selling petroleum or related products.

US officials have warned that depriving global markets of Iranian exports could send oil prices sharply higher, handing Tehran a windfall at a time when it has struggled to cope with painful international economic sanctions.

To address that concern, Kirk and Menendez's measure says the sanctions would only apply if Obama determines that there is sufficient oil from other producers to avoid disrupting global markets, and enables him to waive them for four months if he determines that to be vital to US national security interests.

And Sherman and Cohen's entreaties ran headlong into sharp criticisms from lawmakers impatient with the pace and scope of pressure on Iran and worried time was running short until the US nemesis joins the club of nuclear nations.

"Even though we've given you the tools, you haven't shown us the robust effort, when the clock is ticking, to use that which we have given you," Menendez scolded, referring to past sanctions laws.

Republican Senator James Risch said the administration was wasting its time trying to "torpedo" the amendment, which he predicted would "pass by a very large vote" and warned of an "urgency gap" between the White House and Congress.

"We hear the words, we hear the talk, but we've wanted action for some time and it just hasn't happened," he said.

Senator Richard Lugar, the committee's top Republican, scoffed at concerns that the measure could lead China to break from the fragile international coalition working to isolate Iran.

"They're not taking this very seriously now anyway," he said, dubbing Beijing "thoroughly uncooperative" with existing sanctions and predicting that the new sanctions could prove a "deadly problem for Iran."

And, with the specter of military force as a last option lurking over the debate, Lugar declared "we're going to have to either contend with diplomacy with the Chinese or potential warfare with the Iranians."

Republican Senator Bob Corker asked Sherman whether Washington was "making plans with our friends towards military action" and sending "signals to Iran that, if these sanctions do not work, we really are prepared to use that option?

"It doesn't look to me like that the sanctions, even though there's been some successes, are going to achieve their end prior to the time that Iran actually has a nuclear weapon," he said.

"Iran understands, and they read the newspapers and see what's happening. They understand it is a serious possibility, and we have reason to understand that they believe that," said Sherman.

"In terms of plans and planning, my experience -- I'm sure yours as well -- is the Department of Defense plans for virtually every hypothetical situation there is in the world," she added.