BRASILIA, March 20 (Reuters) – Brazil’s environmental regulator Ibama has asked oil company Petrobras (PETR4.SA) for additional information on its plan to drill at the mouth of the Amazon river before authorizing Petrobras to test its emergency oil spill response, the agency’s president Rodrigo Agostinho told Reuters.

The request frustrated the hopes of Petrobras to start the drilling test as early as this week, a source close to the company said on Monday. Ibama has not defined a test date because Petrobras did not deliver all the documents required, but it will be scheduled as soon as the company provides the information, Agostinho said.

Petrobas views the mouth of the Amazon as the newest and most important frontier for oil exploration in Brazil and the company planned the test to assess its response in the event of a major spill.

The company, which declined to comment for this story, has been working for years to open up a new exploration frontier in a region close to Guyana, where Exxon Mobil (XOM.N) has made important discoveries and many wells were drilled.

The area was auctioned in 2013 and Petrobras has planned to explore there for years after BP (BP.L) and TotalEnergies (TTEF.PA) gave up on their assets, even after investing in studies and due to difficulties in obtaining licenses for drilling.

The Foz region has little known geology and Petrobras has already run up 290 million reais ($55 million) in costs preparing for the spillage test.

But Ibama views the area, known in Portuguese as the Foz do Amazonas, as environmentally sensitive and a decision on the drilling plan cannot be based on business interests and deadlines, Agostinho said in an interview.

“This is obviously a very sensitive region and Ibama treats it as a great priority, so there is no decision on a license or when it can come,” Agostinho said.

He said the previous government “did not carry out a technical assessment of the entire region, which should have been made, and this makes it difficult to assess the situation.”

Agostinho denied that Environment Minister Marina Silva had stopped the drilling plan.

Silva has said exploration at the Foz do Amazonas is “highly impactful” and must have a “strategic” environmental assessment.

($1 = 5.2408 reais)

Reporting by Lisandra Paraguassu and Marta Nogueira; Editing by Chizu Nomiyama and Josie Kao

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