VIENNA - US Secretary of State John Kerry said on Monday Iran and the six world powers had made substantial progress to reach a final deal over its nuclear program, but despite new ideas being presented the talks would remain tough in the coming months."If we can do it (get a deal) sooner, we want to do it sooner," Kerry told reporters. "These talks are not going to get easier just because we extend them. They're tough. They've been tough. And they're going to stay tough."He said that the powers could not keep talking with Iran forever without serious progress, but it was not the time to walk away."In these last days in Vienna we have made real and substantial progress and we have seen new ideas surface," he said, adding there were still "some significant points of disagreement."He said there would be no additional sanctions relief.