By John Irish and Parisa Hafezi

PARIS/DUBAI (Reuters) - Iran will hold talks in Istanbul on Friday with European parties to their now-moribund 2015 nuclear deal, two European and an Iranian diplomatic source said on Tuesday,

The talks among senior diplomats come as both sides seek to position themselves ahead of an expected fifth round of U.S.-Iranian negotiations in the coming days.

Earlier talks planned on May 2 were postponed with Britain, France and Germany, known as the E3, initially hesitant over concerns such talks could create a parallel track and hijack the negotiations pursued by the Trump administration that Washington says aim to prevent Iran from developing nuclear weapons.

But diplomats said they opted to go ahead because it was ultimately in their interest to maintain dialogue with Iran and reaffirm how they envisaged the parameters of a new nuclear deal.

Iran is also keen to hold the talks to keep its options open and assess where the European powers stand regarding the reimposition of U.N. sanctions, something Iran's foreign minister warned them on Monday against doing.

Relations between the E3 and Iran have worsened over the last year despite sporadic meetings, against a backdrop of new sanctions imposed on Tehran over its ballistic missile programme, its detention of foreign citizens and support for Russia in its war against Ukraine.

Under terms of a U.N. resolution ratifying a 2015 nuclear pact, the three European powers could reimpose United Nations sanctions against Tehran before October 18, known in diplomatic circles as the "snapback mechanism."

The European powers are not part of current negotiations between Iran and the United States, the fourth round of which ended in Oman on Sunday.

But the three powers have sought to coordinate closely with Washington with a view to whether and when they should use the snapback mechanism to raise pressure on Iran over its nuclear programme.

According to diplomats and a document seen by Reuters, the E3 countries may trigger a snapback by August if no substantial deal can be reached by then.

The next round of talks between Iran and the U.S. to resolve disputes over Tehran's nuclear programme will be held in the coming days after both sides have consulted with their respective capitals, according to Oman, which mediates the negotiations.

(Reporting by John Irish and Parisa Hafezi; Editing by Rod Nickel)