By Shadia Nasralla
VIENNA (Reuters) - The timing of sanctions relief is the sticking point in nuclear talks between Iran and the six major world powers this week in Vienna, where negotiations kicked off on Wednesday with a meeting between delegates from Tehran and the European Union.
Iran and the six powers are trying to end more than 12 years of diplomatic wrangling over the country's disputed nuclear program, which Tehran says is peaceful but Western governments fear is aimed at developing an atomic bomb.
After a tentative deal between Iran and the P5+1 - China, France, Russia, Britain and the United States, plus Germany - was reached in Switzerland on April 2, differences have emerged over what was agreed.
Arriving in Vienna, deputy foreign minister, Abbas Araqchi, reiterated Iran's position: "All the economic sanctions should be lifted on the day that the deal is implemented," according to a report from Iranian news agency Tasnim.
But the United States has made it clear that sanctions on Iran would have to be phased out gradually under the final pact.
A meeting between Araqchi and EU political director, Helga Schmid, began on Wednesday afternoon. Talks between Iran and delegates from the six powers, including U.S. Under Secretary Wendy Sherman, will follow this week, an EU statement said.
U.S. President Barack Obama was forced to give Congress a say in any future accord - including the right of lawmakers to veto the lifting of sanctions imposed by the United States.
Araqchi said on state television before his arrival in Austria that the U.S. administration was "responsible to ensure that its commitments, particularly sanctions-related ones, are fulfilled".
Efforts have intensified to reach a final deal as a self-imposed deadline at the end of June is looming large for Iran and the P5+1 group.
"We think it is possible to reach a fair and just agreement... by the end of June or even before that," Araqchi said, noting that the Vienna talks are expected to go on for three days.
Iran's foreign ministry spokeswoman Marzieh Afkham said on Iranian state television on Wednesday that U.S. Secretary of State, John Kerry, and his Iranian counterpart, Mohammad Javad Zarif, "discussed Iran's nuclear issue by phone last night".
Zarif was quoted by Iranian media as saying that he would meet Kerry on Monday in New York on the sidelines of the 2015 nuclear Non-Proliferation Treaty review conference being held at the United Nations.
(Additional reporting by Parisa Hafezi in Ankara and Adrian Croft in Brussels; Editing by Louise Ireland)