Nuclear talks with Iran at 'difficult' point: US

"The next phase of this process will be difficult because we will be focusing on the details," a senior US official told reporters.

Washington - With a mere few weeks left to reach a final agreement, nuclear negotiations between the US and Iran are at a "difficult" point, said an official US source.

"The next phase of this process will be difficult because we will be focusing on the details," a senior US official told reporters on Wednesday on condition of anonymity, according to the Wall Street Journal.

"It doesn't help anyone to delay tough decisions as they become tougher over time," he added.

He acknowledged negotiators have been unable to reach a breakthrough and are likely to remain in Vienna till the end of this month.

For the US, it will be "essential" and "crucial" that any pact with Iran must include a large monitoring of Iranian nuclear activities, he added.

Meanwhile, Iran and the P5+1 group of world powers -- the US, France, China, Britain, Russia plus Germany -- renewed on Wednesday efforts to secure a definitive deal on Iran's nuclear programme before the June 30 deadline.

A preliminary agreement was reached last April which requires Iran to commit to a major limitation of its nuclear programme in return for an end to international sanctions, among other things.

According to the US official, the negotiators are working on the demands of Western countries that Iran take concrete steps to clarify its past nuclear activities, that includes trying to develop nuclear weapons on the back of its atomic programme.

"We have a solution, and we are specifying the details of it," he said.

The official also said that US Secretary of State, John Kerry, might join talks in Vienna if required.

Kerry, at present, is recovering from a leg surgery, away from his official schedule.