Brazil's top prosecutor has asked the Supreme Court to open a probe into alleged obstruction of justice by President Dilma Rousseff, Brazilian media reported, in a potentially explosive twist to the country's political crisis.

According to reports in the Globo, Folha de Sao Paulo and Estadao dailies, Chief Prosecutor Rodrigo Janot has requested authority to open an investigation into the embattled president and also her predecessor and key political ally Luiz Inacio Lula da Silva.

Officials could not immediately be reached to confirm the reports, published Tuesday.

But if confirmed, the probe into Rousseff would be on top of a separate investigation that Janot earlier Tuesday asked the Supreme Court to open against Lula and three of Rousseff's ministers in relation to corruption at the state oil giant Petrobras.

Rousseff - who could face the start of an impeachment trial on separate charges as early as next week - allegedly colluded with Lula to obstruct a judge leading a huge investigation into corruption at Petrobras by top politicians and executives, Janot was quoted as stating.

Rousseff's current attorney general, Jose Eduardo Cardozo, is also implicated.

At issue is an attempt by Rousseff to name Lula to a post in her government. Rousseff said she wanted him to lead her defense against impeachment.

But prosecutors, who wire-tapped the president's phone conversation with Lula, said the appointment aimed to obtain ministerial immunity for the ex-president.

Lula, who is fighting corruption charges related to the Petrobras scheme, risks being prosecuted by the main investigating team led by Judge Sergio Moro.

However, if he were in the government, he would gain limited immunity, with only the Supreme Court able to handle his case.

The Supreme Court is now preparing to rule on whether he can take his post.

Janot's filing, if confirmed, would ramp up pressure on Lula and Rousseff.

Prosecutors looking into the controversy have decided that Lula's nomination was part of a "scenario" involving several attempts to obstruct Moro's investigation into the Petrobras scheme, Estadao newspaper reported.

A lower court, headed by a judge in the town of Curitiba, has been leading the Petrobras probe, uncovering what is believed to be Brazil's biggest ever corruption scandal, featuring a network of politicians who took bribes to facilitate inflated Petrobras contracts for crooked construction companies and others.