VIENNA—

U.S. President Barack Obama said Tuesday that the United States would "walk away" from a possible nuclear deal with Iran if he is not convinced it would block Tehran's path to building a nuclear weapon.

"I will walk away from the negotiations if it is in fact a bad deal," he said. "There's still some hard negotiations to take place."

His comments came as negotiations in Vienna on Iran's nuclear program, which were under a Tuesday deadline, were extended for a week.

Obama said at a White House news conference that Iran must agree to a "strong, rigorous verification mechanism" on curbing its nuclear development program.

He said Tehran must live up to a framework agreement it reached earlier this year with the U.S. and five other world powers to allow inspectors to visit Iranian nuclear sites to verify that it is keeping its commitments.

"Ultimately this is going to be up to the Iranians," he said.

"My hope is they can achieve an agreement," he said during a news conference with Brazilian President Dilma Rousseff

Obama's comments came as the U.S., Germany, France, Russia, China and Britain extended negotiations with Iran in Vienna past their self-imposed Tuesday deadline for completing a deal.