VIENNA, Oct 29 (Reuters) -

The U.N. nuclear watchdog's board on Tuesday picked Argentina's Rafael Grossi to become the agency's next chief, and he immediately pledged to act independently and neutrally on issues including Iran.

Grossi, 58, succeeded Yukiya Amano, who died in office.

Grossi had been running neck-and-neck with Amano's former right-hand man and current acting director general, Cornel Feruta of Romania, in repeated votes by the International Atomic Energy Agency's 35-nation Board of Governors until Grossi took the lead on Monday.

Grossi, who had the early support of heavyweights including Brazil and the United States, reached the two-thirds of votes required on Tuesday, with 24 countries backing him. His appointment will now be signed off on by another board meeting and a wider meeting of all member states, both formalities.

"I will do my job and I think my job is to implement the mandate in a manner which is independent, which is fair, which is neutral," Grossi told reporters after the vote when asked about Iran, declining to go into specifics.

Diplomats do not expect a major shift under Grossi in the IAEA's stance on its most high-profile issues, such as policing Iran's deal with major powers, which Iran is breaching step by step in response to crippling U.S. economic sanctions.

"What I think is important is that I give my member states and the international community the guarantee that I am absolutely independent and impermeable to pressure," said Grossi, a veteran of nuclear diplomacy and currently Argentina's ambassador to the IAEA.

The IAEA board aims to have Grossi take office by Jan. 1. Grossi said that might happen earlier.