United Nations (United States) (AFP) - The United States will be isolated on the world stage and its influence diminished if Congress rejects the Iran nuclear deal, the US ambassador to the United Nations has warned.

In an editorial published by Politico, Samantha Power argued that a "no" vote from Congress would make it more difficult for the United States to drum up support for sanctions and partner with like-minded countries to confront crises.

"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," Power wrote in a piece posted late Wednesday.

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

The US Congress is due to vote next month on whether to endorse the deal reached in July between Iran and six world powers -- Britain, China, France, Germany, Russia and the United States.

The agreement would lift crippling UN sanctions against Tehran in exchange for curbs on Iran's nuclear program.

Power argued that rejecting the deal would undermine the US's ability to seek sanctions in other situations because it would convey an image of the United States as "a superpower intent on inflicting pain for its own sake."

The ambassador urged US senators and representatives to carefully consider the fallout of a "no" vote on US diplomacy.

"The price of our lonely walk away looks very high indeed," she said.

Even if the Republican-dominated Congress passes a resolution against the deal, President Barack Obama could still veto that move, but the administration would like to avoid such a scenario.

"If Congress rejects the deal, 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.