The United States will be isolated internationally and will no longer have any influence on the world stage if Congress rejects the Iran nuclear deal, the US ambassador to the United Nations warns.

"If the United States rejects this deal, we would instantly isolate ourselves from countries that spent nearly two years working with American negotiators to hammer out its toughest provisions," Samantha Power wrote in an editorial published by Politico late Wednesday.

Power argued that a "no" vote from Congress may dramatically undercut the US’ ability to achieve other foreign policy objectives it is currently pursuing.

"From this vantage point, I believe that rejecting this deal would significantly weaken our ability to achieve our broader foreign policy goals – most of which in 2015 require us to mobilize broad international coalitions," Power wrote.

"We would go from a situation in which Iran is isolated to one in which the United States is isolated," she asserted.

Iran and the P5+1 group of countries -- the US, Britain, France, Russia, China, and Germany – announced the conclusion of nuclear negotiations in the Austrian capital, Vienna, on July 14. Congress will vote on the July accord, known as the Joint Comprehensive Plan of Action, in September.

Under the agreement, restrictions will be put on Iran’s nuclear activities in exchange for, among other things, the removal of all economic sanctions against the Islamic Republic.

Power argued that rejecting the deal would also undermine Washington’s ability to drum up support for sanctions in other situations.

The reason is, she wrote, that it would convey an image of the US as "a superpower intent on inflicting pain for its own sake."

If Iran accord is rejected, "we will project globally an America that is internally divided, unreliable, and dismissive of the views of those with whom we built Iran's sanctions architecture in the first place," Power warned.

Most Republicans, including at least two prominent Senate Democrats, have opposed the agreement, however, President Barack Obama is confident that “this deal will go forward.”