US Secretary of State John Kerry and US Under Secretary for Political Affairs Wendy Sherman meet with foreign ministers and representatives of Germany, France, China, Britain, Russia and the European Union during nuclear talks at a hotel in Vienna, Austria, July 10, 2015. [Photo/Agencies]

Western states fear that once Iran own advanced centrifuges, Tehran would have greater breakout capability, but Iran sees the research and development of nuclear plan as its nuclear right. US Secretary of State John Kerry made positive remarks over the talks, saying the atmosphere is very constructive, said media reports.

The source said a big challenge now is how to choose the key words in the text, the parties still need time to work on it.

Foreign ministers from world powers are coming back to rejoin the talks in the coming days. Iran and the five UN Security Council permanent members plus Germany have been negotiating over the past 18 moths to reach a long-term deal over Tehran's controversial atomic plan.

On Thursday, Kerry warned US could walk away if no tough decision are made while Iran accused western states of changing their positions in the final days of the talks. Progress was also made in the arms embargo of the UN sanctions on Iran, a heated issue in the resent talks.

"There are a few remaining issue which needed to be resolved," the source said, adding the parties would try hard to bridge gaps in the coming days.