By Fabio Teixeira

RIO DE JANEIRO, May 20 (Reuters) - A decision by Brazilian environmental agency Ibama on Monday will let state-run oil firm Petrobras PETR4.SA move a step closer to drilling for oil in a coveted offshore region, but it came with an important caveat for future permits in the area.

Documents seen by Reuters show Ibama head Rodrigo Agostinho warned in his decision against "disorderly multiplication of future requests for environmental licenses" in the Foz do Amazonas basin, an oil frontier near the mouth of the Amazon River.

The area, in the northernmost part of Brazil's Equatorial Margin, is considered Petrobras' most promising oil frontier, sharing geology with nearby Guyana, where Exxon Mobil XOM.N is developing huge fields.

But Agostinho said it would be hard to issue "fragmented and successive exploration licenses" in the Foz do Amazonas basin without a complex environmental study known as an AAAS that could take years to complete.

Ibama had previously requested an AAAS to evaluate Petrobras' bid, but Brazil's solicitor-general issued a legal opinion that such a study should not hold up licensing.

Agostinho's renewed demand adds to uncertainty about future licensing in the region, where Brazil is preparing to offer new blocks at a June auction.

For now, the agency chief granted approval to a Petrobras proposal for how it would assist local fauna in the event of an oil spill in the environmentally sensitive region, which includes vast coral reefs and coastal Indigenous communities.

Petrobras said it welcomed the decision on Monday, taking it as a green light to run a test of its environmental emergency plan, which it called the last step before a final licensing decision.

The progress for Petrobras represents a loss for Ibama's technical staff, which had signed a document in February saying the plan to rescue fauna in case of an oil spill had only a "remote possibility" of being successful.

In 2023, Ibama denied a Petrobras request to drill in the area, which the company immediately appealed, stoking divisions in Brazil's government between environmental advocates and allies pushing for oil and gas development in the region.

(Reporting by Fabio Teixeira; Editing by Brad Haynes and Jamie Freed)

((Fabio.Teixeira@thomsonreuters.com; +55 21 99603-4782;))