Washington - The United States' top negotiator for Iran nuclear talks made the case to lawmakers Wednesday for sticking with what may be a last try for a deal reimposing limits on Iran's nuclear development, despite Iran closing in on completing a bomb-capable nuclear program.

Rob Malley, President Joe Biden's envoy to negotiations aimed at getting the U.S. and Iran back in a breached 2015 Iran nuclear deal, faced lawmakers on the Senate Foreign Relations Committee.

Some Democrats on the committee joined Republicans in saying it was past time to break off talks and move to tougher means to block Iran from taking the final, technical steps needed to build a bomb.

Biden has made a priority of returning the U.S. and Iran to the nuclear pact, abandoning what his administration says was a failed get-tough strategy by President Donald Trump. Failing to do so would be a politically damaging setback to his foreign policy agenda and risk a dangerous escalation of tensions in the Middle East.

Malley acknowledged to lawmakers the chances of success were 'tenuous' in what the administration depicts as a final push to try close an agreement.

The accord would ease punishing international sanctions on Iran in exchange for Iran accepting limits and oversight of its nuclear work.

Malley underlined to lawmakers that the administration still believed Iran's nuclear program was less of a threat inside a deal than outside of one.

'How long is this going to go on?' Sen. James Risch of Idaho, the committee's top Republican, demanded of the often slow-moving, off and on talks.

The Biden administration is "prepared to get back into the JCPOA for as long as our assessment is that its nonproliferation benefits are worth the sanctions relief,' Malley responded.

Iran jumped back into building its nuclear capacity after Trump in 2018 pulled the U.S. out of the international nuclear deal negotiated with the Obama administration.

The many opponents of the so-called Joint Comprehensive Plan of Action increasingly argue Iran's progress since then toward the ability to weaponize what it says is a civilian nuclear program means it's too late for any accord to block Iran from becoming a nuclear power.

The International Atomic Energy Agency said last week that Iran has amassed about 40 kilograms (about 90 pounds) of uranium enriched to 60% purity, a short, technical step away from weapons-grade levels.

'I think we must prepare for the increasingly obvious reality we face in 2022 - a return to the 2015 nuclear deal is not around the corner, and I believe it is not in the U.S. strategic interests,' committee chairman Bob Menendez, one of the lawmakers of Biden's own party opposed to the Iran nuclear deal, said.

'We need to tackle what comes next,' Menendez said.

Biden has insisted the U.S. will never allow Iran to take the final steps toward attaining nuclear programs.

Malley repeated that the administration and its allies are preparing options if talks fail. The administration has declined to detail publicly what steps it would take to knock offline Iran's centrifuges and other nuclear gear, much of it deep underground and well-defended against airstrikes.

'Being at the table doesn't mean we're waiting. We're not waiting, we're acting,' Malley told lawmakers.

Democratic lawmakers in support of the negotiation effort argued that giving up on peaceful pressure and negotiations now to move to military strikes against Iran's nuclear program would be dangerous, and likely futile.

Sen. Tim Kaine, D-Va., urged U.S. leaders to resist what he called the siren song of the option of Israeli warplanes to move against the nuclear targets of Iran, Israel's top opponent.

'It's difficult to bomb knowledge out of existence,' added Sen. Chris Murphy, D-Conn., suggesting Iran would only rebuild its program after strikes. 'And the risk of spillover of a regional war is significant.'

Malley promised lawmakers on Wednesday that the administration would allow Congress to review any deal that does come out of talks. That answered a demand of Menendez and other hard-liners on the nuclear accord. Lawmakers could try to block any new deal, but could face a presidential veto.

The talks to get Iran and the U.S. back into compliance with the nuclear deal appeared to have reached agreement on all but the final points of the accord itself by late winter. But talks deadlocked since then in part over Iran's demand for the United States to lift a terrorist designation on the country's paramilitary Revolutionary Guard.

European diplomacy over the past several days is believed to have persuaded Iranian leaders to drop that demand. It was a politically untenable one for the Biden administration, which is eager to show U.S. conservatives and ally Israel it remains vigilant against Iran's efforts to build strength and influence in the Middle East.

Even as Malley urged more time for the talks, the Biden administration announced new sanctions Wednesday against Iran, among what Malley said were about 150 sanctions designations targeting Iran by the Biden administration. Wednesday's sanctions hit what the U.S. said was an operation by a current Revolutionary Guard official and a former one to smuggle oil in violation of international sanctions.