VIENNA -- US Secretary of State John Kerry met once again with Iranian Foreign Minister Mohammad Javad Zarif on Wednesday morning in the Austrian capital, hours after both of their bosses issued tough statements on their standards for a comprehensive nuclear deal. Starting the day, Iran's deputy foreign minister, Abbas Araqchi, said that both sides had already shown the will necessary to proceed toward an historic agreement.But for Iran's part, such an agreement cannot come at "any cost," he said. The US, Britain, France, Russia, China and Germany seek to curb Iran's nuclear work for a limited period. Iran seeks immediate sanctions relief and to retain much of its nuclear infrastructure.On Tuesday evening, US President Barack Obama warned Iran against walking back its agreements made in principle in Lausanne, Switzerland, meant to frame a final deal over its nuclear program. “I will walk away from the negotiations if, in fact, it’s a bad deal,” he said. “If the verification regime is inadequate, then we’re not going to get a deal.”Iranian President Hassan Rouhani warned back that, should a deal fail, Tehran would rev up its nuclear work beyond any expectations or fears in the West. Entering his morning meeting with Kerry, Zarif said he had invited the head of Iran's nuclear watchdog, the International Atomic Energy Agency, to Iran on Thursday.He denied that the decision by negotiators to extend their interim agreement— the Joint Plan of Action— until July 7 amounted to a deadline."We did not set any deadline," Zarif said. "We will continue, and we will make progress."Zarif also planned meetings with his counterparts from Britain and Germany.