US Secretary of State John Kerry says nuclear negotiations with Iran are not open-ended but there is no rush for a deal.

“We will not rush and we will not be rushed” to a comprehensive deal, he told reporters in the Austrian capital, Vienna, on Thursday.

“We are not going to sit at the negotiating table forever” he said.

Iran and the P5+1 countries - the United States, Britain, France, Russia, China and Germany - have been engaged in crucial talks in Vienna towards reaching a landmark agreement over Tehran’s nuclear program.

The negotiating sides missed an initial deadline of June 30 and all parties agreed on a new July 7 deadline. However, negotiations will continue for "a couple" of more days as some issues remain unresolved.

Kerry emphasized that progress has been made in the talks but tough issues still remained unresolved.

“Let me assure you we would not be here continuing to negotiate for the sake of negotiating,” he said. “We know that difficult decisions don't become easier over time.”

The top US diplomat said that the US and its negotiating partners are “prepared to call an end to this” if “tough decisions don't get made.”

During a meeting with a group of Democratic senators on Wednesday, President Barack Obama said the chances of a deal with Iran were less than 50-50 at this point, according to one of the senators present at the meeting.

“He [Obama] said… he wouldn’t agree to something he thought was weak or unenforceable,” Senator Dick Durbin told POLITICO.

“But if he comes up with an agreement and it meets his standards, he wanted us to take an honest look at it and not prejudge,” he added.