By Fredrik Dahl

VIENNA (Reuters) - The U.N. nuclear watchdog said on Sunday that the resignation of one of its top officials who have been leading talks with Iran would not change its policy in dealing with Tehran over its disputed atomic program.

The message of continuity came two days after diplomats said that Rafael Grossi would quit as assistant director general of the International Atomic Energy Agency (IAEA), a post which made him one of the U.N. agency's most influential people.

His surprise departure coincides with apparent deadlock in the IAEA's push since early last year to coax Iran into allowing its inspectors to restart a long-stalled investigation into suspected atomic bomb research by the Islamic Republic.

Western diplomats blame Iranian stonewalling for the failure to come to an agreement, a charge Tehran denies, and some say the U.N. agency may soon need to reconsider its tactics. A new round of talks, the tenth, could be held in May.

Some envoys said the Argentine diplomat's sudden resignation announcement was a possible indication of personal or other differences among the leadership of the Vienna-based IAEA.

Other officials, however, said they had seen no sign of any such internal friction in the organization's senior management.

Grossi is expected to leave his post in the early summer to become Argentina's envoy to the IAEA, a diplomatic source told Reuters on Friday. The South American country is a member of the U.N. agency's 35-nation governing board.

The IAEA confirmed on Sunday that Grossi, who also holds the position as chief of cabinet of IAEA Director General Yukiya Amano, had handed in his resignation and that the "timing of his departure will be defined very soon."

Spokeswoman Gill Tudor said that Amano "confirms this departure will not change the positions and policies of the agency, including the on-going negotiations with Iran."

SUSPECTED BOMB RESEARCH

Grossi is one of two senior IAEA officials who have been leading the agency's efforts since early 2012 - so far in vain - to persuade Iran to give its inspectors access to sites, officials and documents for their inquiry.

His resignation means that both of them will leave the agency this year: the IAEA said last month that a senior Finnish official, Tero Varjoranta, would succeed chief IAEA nuclear inspector Herman Nackaerts when he retires in the early autumn.

But analysts and diplomats stress that it is Amano who decides policy. He steered the agency into a tougher approach to Iran and secured a second four-year term in March.

"It will be business as usual at the agency," one diplomat in the Austrian capital predicted.

Mark Fitzpatrick of the International Institute for Strategic Studies, a think tank in London, said Amano would not select replacements who "differed in substance" from Nackaerts and Grossi.

"The institutional norms and ways of doing business don't easily change," Fitzpatrick said.

In late 2011, the IAEA published a report with a trove of intelligence indicating past research in Iran which could be relevant for nuclear weapons, some of which might still be continuing. Iran dismissed the findings as baseless or forged.

The IAEA-Iran talks are separate from, but still closely linked to, broader diplomatic negotiations between Tehran and six world powers aimed at resolving the decade-old dispute peacefully and prevent a new Middle East war.

Iran denies Western allegations that it is seeking to develop the capability to build nuclear weapons, saying its atomic activities are aimed at generating electricity.

But its refusal to curb sensitive nuclear work that can have both civilian and military purposes and its lack of openness with IAEA inspectors have drawn tough Western sanctions.

(Editing by Stephen Powell)