The United States, Iran and the European Union are having a trilateral meeting on Monday in Vienna after the bilateral meeting between U.S. Secretary of State John Kerry and Iranian foreign minister Mohammad Javad Zarif, a senior U.S. Official said.

Racing with the clock, the intensive talks are trying to seize the historical opportunity for reaching a comprehensive solution to put an end to the decade-old Iranian nuclear row.

Following the foreign ministers' meeting on Sunday, Kerry and Zarif held bilateral talks in Vienna to discuss a possible deal by the deadline July 20 when the Geneva interim deal reached November last year is to expire.

Under the interim deal, Iran agreed to suspend some sensitive nuclear activities in exchange for limited sanction relief by western powers in a duration of six moths to buy time for the negotiation aimed at a comprehensive solution to resolve the issue.

The meetings of foreign ministers from P5+1 group (five permanent members of the United Nations Security Council -- the United States, Britain, France, Russia and China, plus Germany) on Saturday failed to bridge the gaps in the talks, but noted the talks are in the critical moment, and would continue working on the issue.

Significant gaps remain on some key issues especially over Iran's uranium enrichment plan.

Iran's supreme leader Ayatollah Ali Khamenei said on Tuesday Iran needs 190,000 separative work units (SWUs) for uranium enrichment, but western states want Iran to only keep a much lower enrichment capacity.

Kerry warned earlier this month that if there is not enough progress in the talks by July 20, the United States may not allow them to continue.

"Obviously we have some very significant gaps still," said Kerry earlier when he arrived in Vienna Sunday to join the talks. "So we need to see if we can make some progress."

Iran's deputy negotiator Abbas Araqchi told Al-Alam television, "On practically all the important issues differences persist and we have not been able to narrow them."

Araqchi said Iran was ready to walk away from the talks if the world powers pushed on with "excessive" demands.

Kerry will "gauge Iran's willingness" and urge his Iranian counterpart Javad Zarif to make "critical choices" during the second day of meeting, a senior State Department official said.

Iran currently has installed 19,000 centrifuges, of which 10,190 are operating.