A top US Republican said a potential historic nuclear deal with Iran would be a "hard sell" in Congress, as talks neared their final hour in Vienna on Sunday.

"It appears as if the administration's approach to this was to reach whatever agreement the Iranians are willing to enter into," US Senate Majority Leader Mitch McConnell said in an interview broadcast on "Fox News Sunday."

"I think it's going to be a very hard sell -- if it's completed -- in Congress," McConnell said.

Talks between world powers and Iran were entering what France described as the "final phase" Sunday, but Washington warned major issues must still be overcome ahead of the latest deadline on Monday for an agreement.

The talks seek to nail down a deal curbing Iran's nuclear activities to make it extremely difficult for Tehran -- which denies any such goal -- to develop the atomic bomb.

In return Iran will be granted staggered relief from painful sanctions, although the six powers insist on the option of re-imposing the restrictions if Tehran breaches the deal.

"No deal is better than a bad deal. And from everything that's leaked from these negotiations, the administration's backed away from almost all of the guidelines that they set up for themselves," House Speaker John Boehner said in an interview that aired on CBS's "Face the Nation."

Republican Senator Tom Cotton furthered Boehner's remarks stating that he thought any deal could be potentially hazardous.

"I think whatever deal comes out of this weekend, it's going to be dangerous for the United States and dangerous for the world," Cotton said.

Senator Lindsey Graham, a Republican contender for the 2016 presidential race, meanwhile said he would use military force if Iran went too far with its nuclear program.

"If Iran gets a nuclear weapon, they're the biggest threat to the world. And I would not allow that to happen, and they need to know that," Graham said.

However, he added: "The interim deal has worked better than I thought it would. Hats off to John Kerry."

From across the aisle, Democrat Bob Menedez echoed the Republicans' worries, stating that the United States was "not rolling back Iran's elicit nuclear infrastructure, but rolling back sanctions for verification."

"I'll judge the agreement based upon what it is. But we have to make very clear that there is a deterrence in the longer term," he said.