Top US and Iranian diplomats warned Saturday that it would take "hard work" to nail down a nuclear deal as talks got underway in the Austrian capital.

"We've a lot of hard work to do. There are some very tough issues," US Secretary of State John Kerry said, as he met with his Iranian counterpart Foreign Minister Mohammad Javad Zarif. The Iranian minister added that "we need to work really hard in order to be able to make progress and move forward."