"Negotiators are expected to be "close" to an agreement on the June 30, if they "can get there at all,"a senior US administration official was quoted as saying.
High-level officials from P5+1 countries are also expected to attend the meet in Vienna.
China, France, Russia, the United Kingdom, and the United States, plus Germany are called P5+1.
The official said negotiators remained "committed" to the Tuesday deadline but may "miss it by a short bit" in order to ensure a proper deal is reached.
"If it takes us a little bit past June 30 to have the right content, as I said a moment ago, what matters here is the substance of the deal and we have to get it right," the official said.
Most difficult issues like the pace of sanctions relief for Iran and "details about access and transparency to be discussed in the meeting.
Iran reached a framework nuclear deal with the P5+1 on April 2. Negotiators are now seeking a final agreement that would restrict Iran's nuclear program in exchange for sanctions relief.
Iranian officials favor an immediate lifting of sanctions, if an agreement is reached, while P5+1 negotiators favor phased relief.