Israeli Prime Minister Benjamin Netanyahu has declared that the deal being negotiated between world powers and Iran will leave Tehran free to develop nuclear weapons.

In an impassioned address to the US Congress, conducted even as Secretary of State John Kerry was in nuclear talks in Switzerland with his Iranian counterpart, Netanyahu branded Iran a global threat.

"That deal will not prevent Iran from developing nuclear weapons," he said, placing himself in stark opposition to US President Barack Obama's policy of containing Iranian ambitions through a diplomatic accord.

"It would all but guarantee that Iran gets those weapons, lots of them," he said, dismissing promises from US and European leaders that the planned accord would block Iran's path to the bomb. "Iran's nuclear program would be left largely intact. Iran's breakout time would be very short," he said.

Around 50 Democratic members stayed away from the event, but many more lawmakers from both sides of the aisle did attend, and Netanyahu was welcomed with a warm standing ovation and left to cheers and raucous applause.

"I know that my speech has been the subject of much controversy. I deeply regret that some perceive my being here as political," he said, acknowledging complaints that he has endangered Israel's close ties with Washington.

He has also been accused of using the nuclear stand-off as a platform for electioneering, two weeks ahead of Israeli polls that could threaten his ruling coalition.

"That was never my intention," he protested. "I want to thank you, Democrats and Republicans, for your common support for Israel year after year, decade after decade. We appreciate all that President Obama has done for Israel," he said, citing the close security co-operation between the countries.

But, despite his warm opening words, his speech built to a thorough denunciation of Obama's efforts to strike an accord, citing Iranian leaders' threats to "annihilate" Israel and "aggression" against their Middle East neighbours.