Brazil seeks mediation role as Ahmadinejad visits

* Brazil aims to facilitate U.S.-Iran talks

* Sees growing role for its diplomacy in Middle East

* Concern that Brazil should take tougher stance

By Stuart Grudgings

RIO DE JANEIRO, Nov 19 (Reuters) - Brazil aims to burnish its growing diplomatic stature by pushing Middle East peace next week as it welcomes Iranian President Mahmoud Ahmadinejad just two weeks after a visit by Israel's president.

For Ahmadinejad, the visits to Brazil and its South American neighbors Venezuela and Bolivia offer a chance to expand Iran's ties in a region where he already has a firm ally in Venezuela's leftist President Hugo Chavez.

Brazil, which has taken a much more conciliatory line over Iran's nuclear ambitions than Western allies, says talking to Iran is more likely to lead to progress on the nuclear issue and Middle East peace than isolating it.

But President Luiz Inacio Lula da Silva's decision to open a dialogue with Ahmadinejad also carries risks if Brazil is seen as failing to press Western concerns over its nuclear program and complaints of human rights abuses.

The dialogue has the backing of U.S. President Barack Obama, who Brazilian government officials say asked Lula to talk with Ahmadinejad during a meeting in London in March.