Negotiators trying to rein in Iran's nuclear program have decided not to set a new target date to conclude talks, France's foreign minister said Monday, and he warned that if an agreement isn't "very robust" other countries in the Middle East will seek to develop nuclear weapons.

"We have made some progress, but still it is not the end of the process," Laurent Fabius told a group of reporters, stressing that all parties have adopted the principle that "nothing is agreed until the moment when everything is agreed."

Fabius said the negotiators decided Sunday that the talks will go past their June 30 target date to wait for answers to outstanding questions. He said he expects foreign ministers from the seven countries to return to Vienna this week.

Russian Foreign Minister Sergey Lavrov will arrive in Vienna on Tuesday and will meet with U.S. Secretary of State John Kerry. Iranian Foreign Minister Mohammad Javad Zarif also is expected in Vienna on Tuesday.

The United States and its Western allies believe Iran is seeking to build nuclear weapons. Iran insists its nuclear program is purely peaceful, to produce nuclear energy.

Fabius said that for France, which is widely viewed as taking a hardline position in the talks, there are three conditions that must be met:

  •   Limitations on Iranian nuclear research and production where "some progress has been made but it still has to be completely agreed."
     
  • The International Atomic Energy Agency, the UN's nuclear watchdog, must be able to verify any agreement.
     
  •   If there is an agreement, sanctions on Iran will be lifted, but if it is not implemented there will be "a snapback mechanism" that will automatically bring back sanctions, "and we are discussing that."

Fabius said the Iranians have specific questions about how the lifting of sanctions will work "and some other elements." He said the six major powers negotiating with Iran -- the U.S., Russia, China, Britain, France and Germany -- are also "asking some precision," but he did not elaborate.

Fabius refused to address questions about possible backtracking by Iran on previously agreed provisions. To a question on whether France is confident that Russia and China would be willing to suspend their Security Council veto power to allow sanctions to automatically be reimposed if an agreement is violated, he said, "we have very good co-operation with the Chinese and Russians."

The French minister stressed repeatedly that any agreement must be "serious and robust."

If it isn't, Fabius warned, some countries in the region will pursue nuclear weapons. He called that "very dangerous" for the region and the world.