Top US diplomat John Kerry said Tuesday he is preparing to resume tough nuclear talks with Iran only weeks after breaking his leg, as Washington works to nail down a deal putting an atomic bomb out of Tehran's reach.

Appearing by live video feed from his home in Boston where he is recovering after surgery and almost two weeks in hospital, Kerry said he would fly back to Washington later Tuesday to plan key talks with China early next week.

"That will be the prelude to my departing at some point -- we're not exactly sure of the date" for the next talks with his Iranian counterpart Mohammad Javad Zarif, possibly in Vienna, he said.

Kerry said he hoped that trip "would be the closeout and should be the closeout of the negotiations with respect to the Iran nuclear program."

The stakes remained "very high," Kerry told reporters, adding the talks were "tough."

"As I have said consistently, we're not going to rush to an agreement for the sake of an agreement, and we're not going to sign an agreement that we don't believe gets the job done."

Iran and the so-called P5+1 group leading the negotiations to stop Tehran developing a nuclear bomb are working towards a June 30 deadline for a final deal.

But one of Iran's top negotiators, Abbas Araghchi, hinted Tuesday the talks may run long as he prepared to head to Vienna for more lower-level meetings.

"The date... was selected for the end of negotiations but we will not sacrifice a good agreement for the sake of the schedule," Araghchi was quoted as saying by Iranian state television.

One of the thorniest issues still being haggled over is the lifting of a web of US, EU and UN sanctions and what to do if Iran violates any deal.

President Barack Obama "has been very clear from the very beginning that we cannot allow a procedure for snap-back to be left in the hands of Russia or China," US ambassador to the UN Samantha Power told US lawmakers earlier Tuesday.

Iran has called for an immediate end to the crippling sanctions. But it is more likely that the deal will include the gradual lifting of certain measures in return for action by Tehran in meeting goals to diminish its nuclear capability.

- Snap-back panel -

Western diplomats say a mechanism is being finalized for how to put the UN sanctions back in place if Tehran violates the deal, which would likely include a panel of the six members of the P5+1 as well as Iran.

But many of the P5+1 -- Britain, China, France, Germany, Russia and the US -- don't want the panel to be held hostage to the same veto power as exercised at the UN, where a lone country can thwart an action by vetoing it.

"We will retain the ability to snap back multilateral sanctions in place without Russian or Chinese support," Power said, adding that Obama would not accept a deal that does not provide for proper access in Iran to ensure compliance.

Iran has long insisted its atomic program is only for civilian energy purposes.

But the UN nuclear watchdog, the IAEA, remains dissatisfied with unanswered questions on past possible military dimensions to the program.

"We have absolute knowledge with respect to certain military activities they were engaged in," Kerry said.

"It's critical to us to know that going forward, those activities have been stopped and that we can account for that in a legitimate way."