Chinese Foreign Minister Wang Yi (R) waits prior to the start of a world powers representatives meeting, to pin down a nuclear deal with Iran, March 30, 2015, in Lausanne, Switzerland. [Photo/IC]

LAUSANNE, Switzerland - Negotiations over Iran's nuclear program reached a critical phase Monday with diplomats struggling to overcome substantial differences just a day before a deadline for the outline of an agreement.

With Tuesday's target date for a framework accord just hours away, the top diplomats from the five permanent members of the UN Security Council and Germany were meeting with Iran to try to bridge remaining gaps and hammer out an understanding that would serve as the basis for a final accord to be reached by the end of June.

"We have achieved some strong common ground during the past efforts and there are only a few problems left blocking our way," Chinese Foreign Minister Wang Yi said, when commenting on the progress of Sunday and Monday, during which intensive minister meetings were held.

He added he was still optimistic for finding a solution for the remaining balks.

According to the Foreign Minister, there might be some "outcomes" Monday night or Tuesday. He said he hoped "the outcomes will be good ones".

"We are working late into the night and obviously into tomorrow," said US Secretary of State John Kerry, who has been meeting with his Iranian counterpart Mohammad Javad Zarif in Lausanne since Thursday in an intense effort to reach a political understanding on terms that Iran would curb some sensitive nuclear activities in exchange for sanctions relief.