Vienna- Talks on Iran's nuclear program are entering a crucial weekend in Vienna but there is no sign of a breakthrough as Monday's deadline looms.

The US admitted that "serious gaps do remain." But Secretary of State John Kerry did cancel plans to leave Austria, and held further talks with his Iranian counterpart Javad Zarif late on Friday.

Six world powers want Iran to curb its nuclear program in return for the lifting of UN sanctions. The US, the UK, France, Germany, Russia and China want Iran to demonstrate it is not seeking to build nuclear weapons, in the face of protestations by Tehran that its program is purely energy-related.

"We are running against the clock," White House spokesman Eric Schultz was quoted as saying in Washington. "Obviously, the deadline is Monday, and our folks there (in Vienna) are working furiously to meet it. Serious gaps do remain."

After meeting Mr Kerry on Friday evening, Mr Zarif said he heard "no new ideas" and had received "no remarkable proposals to take to Tehran."

The aim is to turn an interim accord reached with Tehran a year ago into a lasting agreement. Iran has been resisting efforts to scale back its nuclear program for nearly a decade.

Separately, the international nuclear watchdog, the International Atomic Energy Agency (IAEA), is calling on Iran to address its concerns about suspicious military activities that could be linked to a nuclear weapons program.

Iran had promised to give the IAEA a response by late August to allegations concerning explosives tests and other activity but has failed to do so and has also denied access to a key military site, Parchin.

UK Foreign Secretary Philip Hammond called on Iran to show "flexibility". At the same time, he warned "We have a long way to go if we are to get to a deal before the deadline." Russian Foreign Minister Sergei Lavrov said that "all the elements are already on the table" as long as there was "political will".

The deal being sought by the world powers is not directly linked to the IAEA's investigation of Iran. However, US said in the past that Iran must address the watchdog's concerns if it expects a comprehensive agreement in the talks.