A pending visit to Tehran by the EU coordinator for talks to revive the Iran nuclear deal cannot be deemed "business as usual" European diplomats have said.

Enrique Mora, the EU's political director and lead nuclear negotiator, is due to hold talks on Thursday with members of Iran's nuclear negotiating team nearly four months after talks broke off between Iran and world powers, including the United States, to rescue the accord.

"The visit comes at an important time," the diplomats from Britain, Germany and France, known as the E3, said in a note on Wednesday. The three countries along with China and Russia are parties to the deal.

"The situation in the nuclear field has been worsening and been aggravated continuously since then," they said, alluding to Iran's accelerating enrichment of uranium to higher fissile purity, a possible pathway to a nuclear bomb.

"For this reason, we do not see this visit as 'business as usual', but rather as a decisive visit in the crisis."

The Islamic Republic has repeatedly said it will return to the negotiations "soon" without giving any sense of what that actually means. Western diplomats have tentatively said a return to the Vienna talks may be possible before the end of October.

Reporting by Reuters