Secretary of State John Kerry appealed to skeptical members of Congress on Tuesday to hold off on imposing new economic sanctions on Iran for at least six months while the Obama administration continues nuclear negotiations with Tehran.

"Six months will fly by so fast, my friends," Kerry said, noting that the deal includes an assurance to Iran that the United States would not impose new sanctions during the six-month negotiating period. "Once implemented ... this agreement halts the progress of Iran's nuclear program, and rolls it back in certain places for the first time in nearly 10 years.”

The House passed legislation that would tighten Iranian sanctions in July on a broad, bipartisan vote of 400 to 20. Democratic and Republican Senate leaders indicated their interest in tightening sanctions as well, and Kerry's appearance before a House committee represented an effort to stem the support among lawmakers for stricter sanctions.

The deal struck between Iran, the United States and other world powers would actually loosen economic restrictions on things like oil sales to allow Iran to pump about $7 billion into its economy. Kerry framed that concession as "temporary, reversible relief" that could be revoked if Iran doesn't negotiate a long-term deal in good faith.

In return, Iran agreed to give international inspectors greater access to certain nuclear facilities. It would also have to stop enriching uranium above a certain level and would not be allowed to increase the number of centrifuges it's operating, all steps to stem the Islamic republic's march to a nuclear bomb.

Members of Congress pressed the nation’s top diplomat on the arrangement, and House Foreign Affairs Committee Chairman Ed Royce, R-Calif., expressed a common concern: That the United States "may have bargained away our fundamental position ... that Iran should not be enriching and processing" uranium even under the limited conditions of the six-month deal.

Kerry allowed that it is not clear whether Iran has changed it calculus on the need for a nuclear weapon, telling the committee, "I honestly don't know if we can say for sure yet."

But Kerry also repeatedly emphasized that the United States was wary of Iran, and that the deal was not based on trust but on repeated verification of Iran's intentions.

"There is no benefit of any doubt here. This is a very skeptical and tested process, verifying a program that we have to account to the world for," he said. If Iran reneged on its obligations, he said, the Obama administration would be the first to ask for stronger sanctions.