Top US Republicans and some Democrats expressed skepticism about Tuesday's historic nuclear deal with Iran, saying it gives Tehran too much room to maneuver and does not safeguard American security interests.

Some in Congress have already said they are prepared to reject the deal because it does not comprehensively halt Tehran's enrichment process or guarantee anytime-anywhere inspections.

And Republican presidential contenders looking to succeed Barack Obama slammed the accord as consolidating the power of Iran's mullahs instead of standing up to them.

"This isn't diplomacy -- it is appeasement," said 2016 candidate Jeb Bush.

Now that Obama has announced his support for the agreement between six world powers and Iran, finalized in Vienna after marathon talks, the attention in Washington shifts to the Republican-controlled Congress.

House Speaker John Boehner blasted the deal as "unacceptable," saying that if it is as he understands it to be, "we'll do everything we can to stop it."

He warned it would "embolden" Iran and could even trigger a global nuclear arms race.

Under legislation passed in May, lawmakers will have 60 days to conduct their review.

Public debate will occur during hearings, possibly beginning next week. Lawmakers are also calling for classified briefings from officials on technical aspects of the agreement.

Congress could then vote to approve or reject it -- or do nothing.

Obama has said he would veto a resolution of disapproval. Overriding that veto would require a two-thirds majority in both the Senate and House of Representatives -- a heavy lift in Congress.

Obama is barred from waiving any Iran sanctions during the review period. And should Congress later determine that Tehran failed to abide by the agreement, it could reinstate sanctions lifted by the president.

- 'Deep skepticism' -

Republicans lined up behind Boehner to voice their concerns.

"I begin from a place of deep skepticism that the deal actually meets the goal of preventing Iran from obtaining a nuclear weapon," said Senate Foreign Relations Committee chairman Bob Corker.

Congress "will need to scrutinize this deal and answer whether implementing the agreement is worth dismantling our painstakingly-constructed sanctions regime that took more than a decade to establish," he said.

Senior Senate Democrat Dianne Feinstein, who once chaired the intelligence committee, however urged lawmakers to study the deal carefully before rendering snap judgments.

"From what I see, it's about as good as we're ever going to get," Feinstein said.

Members from both parties stressed it will be a tough sell in Congress.

"I'm concerned the red lines we drew have turned into green lights, that Iran will be required only to limit rather than eliminate its nuclear program, while the international community will be required to lift the sanctions," warned Senate Democrat Robert Menendez, an architect of stiff sanctions against Tehran.

"The bottom line is: The deal doesn't end Iran's nuclear program -- it preserves it."

Despite the broad skepticism, it would ultimately be difficult for Republicans to persuade enough Democrats to abandon Obama and block the accord.

"It's highly unlikely that Congress would reject the deal with a veto-proof majority," Larry Hanauer, a policy analyst with Rand Corporation and former staff director of a House counter-terrorism panel, told AFP.

While Republican presidential candidates including Senator Marco Rubio and Donald Trump blasted the deal, Democratic frontrunner Hillary Clinton expressed cautious support.

"I think this is an important step that puts the lid on Iran's nuclear programs," Clinton told reporters after meeting with Democratic lawmakers in Washington.

Some Democrats openly questioned the motives of Republican candidates who were expressing full-throated opposition to the deal before reading it.

"It's embarrassing the Republicans are standing up and saying they're against it when the ink is hardly dry. We want to read it and then take a thoughtful position," said Dick Durbin, the number two Democrat in the Senate.

Several lawmakers warned that the deal rewards Iran with billions of dollars in sanctions relief.

Republican Senator Lindsey Graham, also a presidential candidate, said Obama and Secretary of State John Kerry have been "dangerously naive" in their dealings with Tehran.

"You have taken the largest state sponsor of terror on the planet and given them money to increase their terrorist activities," such as funding groups like Hezbollah and Hamas, as well as Syrian President Bashar al-Assad, Graham said.