As talks on an Iran nuclear deal enter the final stretch, U.S. lawmakers are sharpening warnings against a "weak" agreement and laying down red lines that, if crossed, could prompt Congress to trip up a carefully crafted international pact.

Several influential lawmakers said they do not want to see any sanctions lifted before Tehran begins complying with a deal, and want a tough verification regime in which inspectors could visit Iranian facilities anytime and anywhere.

They also want Tehran to reveal past military dimensions of its nuclear program, particularly after Secretary of State John Kerry seemed to soften the U.S. stance last week by saying Iran would not be pressed on this point.

"I have become more and more concerned with the direction of these negotiations and the potential red lines that may be crossed," Senator Bob Corker, the Republican chairman of the Senate Foreign Relations Committee, told a hearing on Wednesday.

Corker authored a bill giving Congress the right to approve or disapprove any final deal that emerges from talks between six major powers and Iran. Kerry travels to Vienna on Friday for the latest round.

The talks, which are expected to drag past a self-imposed June 30 deadline, could mark the end of a nearly two-year negotiating process aimed at restricting Iran's nuclear program in return for sanctions relief.

President Barack Obama signed the bill into law last month after the White House failed to persuade enough Democrats not to join Republicans in demanding a say.

As the deal deadline nears, lawmakers are coming under pressure not to support an agreement that gives much ground to Tehran.

AIPAC, the influential pro-Israel lobby, has been campaigning hard in Congress on its concerns that any agreement could be "fundamentally flawed." J Street, a more moderate pro-Israel group, has launched its own campaign rebutting arguments made by opponents of a deal.

Several other groups, including United Against a Nuclear Iran, and the American Security Initiative, founded by three ex-senators, are spending millions of dollars on advertising campaigns urging lawmakers to take a hard line.

"There is tremendous skepticism about this deal ... and some Democrats from heavily pro-Israel communities are going to have a tough time with this," Republican Senator John McCain said.

A group of prominent American security advisers, including five with ties to Obama's first term, warned in an open letter on Wednesday that a deal was at risk of failing to provide adequate safeguards.

Obama would likely veto any Congressional resolution of disapproval, which would require 60 votes in the Senate and a majority in the House.

To get the two-thirds majority in both houses of Congress to override a veto, opponents would need at least 13 Democrats in the Senate and at least 43 in the House, and more if they are not supported by every Congressional Republican.

That appears unlikely, but significant weaknesses in the final pact would make it less so, lawmakers from both sides said. The Congressional demands for a watertight deal put U.S. negotiators under additional pressure not to give Iran much leeway.

Virginia Senator Tim Kaine, a Democrat, said a deal would be a "non-starter" for him if, for example, Iran refused to allow inspections on military bases.

"The two biggest issues for people will be the intrusive nature of the inspections and how comprehensive they are, and the timing of sanctions relief," he said.

Western officials say inspections of military sites and access to Iran's scientists are critical to checking whether Iran is pursuing a clandestine nuclear weapons program.

HEIGHTENED CONCERNS

Concerns on Capitol Hill were heightened on Tuesday when Iranian Supreme Leader Ayatollah Ali Khamenei ruled out inspections of military sites, rejected freezing sensitive nuclear work for a long period and said sanctions should be lifted when a deal is reached.

"It would be better if there were encouraging statements coming out of Tehran," said Representative Eliot Engel, the top Democrat on the House Foreign Affairs Committee.

"Then we would feel at least that maybe they do want to change their attitudes and maybe we should change some of our attitudes, too, but I haven't seen it."

Corker's "Iran Nuclear Review Act of 2015" passed the Senate by 98-1 and the House by 400-25.

The Obama administration has until July 9 to transmit a final nuclear deal to Congress, triggering a 30-day period in which the Senate and House can consider a resolution approving it, vote on a resolution of disapproval or have no vote at all.

The measure bars Obama from waiving any sanctions on Iran approved by Congress during the review period, plus 22 days if Congress passes a disapproval resolution and Obama vetoes it.

If a resolution of disapproval survived a veto, Obama would be barred from waiving Congressional sanctions. Since those account for the vast majority of U.S. sanctions, it could cripple any nuclear deal.

The review period doubles to 60 days if Congress gets a deal between July 10 and Sept. 7, increasing the time pressure on negotiators. If there is no deal by Sept. 7, lawmakers would seek to pass additional sanctions.

Several Democrats said they were confident most of their party would be comfortable approving a deal, if it were largely similar to a framework pact announced in April.

But they acknowledge any pact will be a tough sell to most Republicans.

"The majority of Republicans are going to vote against anything that has President Obama's signature on it," Democratic Senator Chris Murphy said.

By Reuters