2nd LD Writethru: Some tough issues remain unresolved: Kerry

U.S. Secretary of State John Kerry on Thursday said some of the tough issues remain unresolved in the ongoing nuclear talks here, warning the United States could walk away from the negotiation if decisions are not made.

As the deadline of the ongoing nuclear talks approaching, difficult choices are needed to be made by sides involved in the talks for a comprehensive Iranian nuclear deal in the capital city of Austria, said Kerry.

"Some of the tough issues remain unresolved... We know that difficult decision don't become easier over time,those decisions must be taken very soon," Kerry said.

Kerry warned that the United States could walk away from the negotiation room if "tough decisions" could not be made.

"We are not going to sit at the negotiation table forever," Kerry warned, saying if the tough decisions will not get made, the United States is "absolutely prepared" to get to an end of this process.

"You can't wait for ever for the decision to be made," he said.

However, the United States said the quality of the deal is more important than the racing with the clock, as the historical deal must be tested by at least a decade of years.

"We shouldn't get up and leave simply because the clock strikes midnight," Kerry said, stressing "the stake (of the talks) is very very high," he said.

"We will not rush and we will not be rushed," Kerry said, "all we are focused on is the quality of the agreement."

It's still unclear how long would the nuclear talks last in the Palais Coburg hotel. The Chinese and Russian foreign ministers are not in Vienna for the moment.

Responding to reporters, Iranian Foreign Minister Mohammad Javad Zarif said he would stay in Vienna as long as necessary.

"I will stay as long as necessary," Zarif told reporters from the balcony of the hotel. Endit