Hours after Iran and major powers extended their talks on a final agreement, an official said talks had to wrap up in the next 48 hours.
The official made clear that the latest extension until Friday would be the last one.
"We've come to the end," the source told Reuters, on condition of anonymity. "We have just made one, final extension. It is hard to see how or why we would go beyond this. Either it happens in the next 48 hours, or not."
Diplomats said a discussion on Monday night between Iran and the major powers became testy over the issue of UN sanctions, which Iran wants scrapped as part of a deal to curb its nuclear program.
"There was no slamming of doors but it was a very heated exchange of views," a senior Western diplomat told reporters, according to Reuters.
The comprehensive deal under discussion is aimed at curbing Tehran's most sensitive nuclear work for a decade or more, in exchange for relief from economic sanctions that have slashed Iran's oil exports and crippled its economy.
Tuesday’s extension marked the fourth time the parties have extended the interim deal struck in November 2013, which gave Iran limited sanctions relief in return for its restricting its nuclear program, including halting production of 20 percent enriched uranium.
The latest extension to Friday leaves open the possibility that an agreement will not arrive in time for a deadline set by the U.S. Congress in order to provide an expedited, 30-day review.
That deadline is set for the end of the day on Thursday in Washington.
Regarding inspections, which Iran has refused to allow on sensitive nuclear sites, an adviser to Iran's parliament speaker on international affairs revealed Tuesday that Iran remains belligerent in its defiance of requests to allow inspections of the facilities.
"That we do not allow our military and sensitive sites to be inspected or that we refuse to send our (nuclear) scientists under the knife of interrogation is part of Iran's obvious and inalienable rights," Hossein Sheikholeslam told the Iranian Tasnim News Agency.
Another key sticking point aside from inspections has been Iran's demand that all sanctions be lifted as soon as a deal is reached, and its refusal to disclose the military aspects of its nuclear program.
On Monday, Iran demanded yet another concession in nuclear talks - this time, to end the UN arms embargo on the Islamic Republic.
Russian Foreign Minister Sergei Lavrov said Iran's opposition to sanctions stems on its desire to have free weapons access, noting, "I can tell you that there is only one big problem in terms of sanctions - it is the problem of a weapons embargo."