"Major issues" must still be resolved in talks between world powers and Iran on a nuclear deal, a US official said Sunday following signs that an agreement may be close.

"We have never speculated about the timing of anything during these negotiations, and we're certainly not going to start now -- especially given the fact that major issues remain to be resolved in these talks," the senior State Department official said.

Iranian diplomat Alireza Miryousefi, writing on Twitter, quoted a senior official from Tehran as saying a deal by Sunday night was "logistically impossible" as the agreement being drawn up spanned 100 pages.

The so-called P5+1 group -- Britain, China, France, Germany, Russia and the United States -- faces a Monday deadline to reach a deal to end a 13-year standoff with the Islamic republic, which is seeking relief from crippling sanctions.

The talks are aimed at finalising a deal to make it extremely difficult for Iran to develop nuclear weapons.

Tehran denies its nuclear programme has military aims.