New York state's top legal officer filed a court motion on Monday to compel former Merrill Lynch CEO John Thain to discuss $4 billion in employee bonuses, ratcheting up its probe into a hot-button issue in the recession.

New York Attorney General Andrew Cuomo alleged that Thain, who was ousted from Bank of America Corp in January just three weeks after the companies merged, said Bank of America told him not to discuss the 2008 bonuses.

Bank of America directed Mr. Thain not to discuss specific bonus details. Mr. Thain continues to cooperate fully with the attorney general's office, said Jesse Derris, spokesman for Thain.

Bank of America, Mr. Thain's former employer, has no authority to issue such an instruction, Cuomo's filing in New York State Supreme Court said.

Cuomo's office alleged that Bank of America was obstructing its probe into bonuses that the attorney general said had made millionaires out of hundreds of employees at the Merrill brokerage amid huge losses.

Cuomo began questioning bonus plans in October after nine banks received $125 billion of U.S. Treasury capital.

Bank of America's instruction to a former employee without basis to refuse to answer questions about the determination and payment of particular bonus awards has no legitimate basis and obstructs the Attorney General's ability to exercise its obligations to investigate and enforce violations of New York's Martin Act, Cuomo's motion said.

Accordingly, the Court should compel Mr Thain to answer the questions.

The Martin Act gives the state's top lawyer extraordinary power in fighting financial fraud.

The motion filed on Monday follows a February 19 deposition of Thain by lawyers from Cuomo's office. The attorney general alleged that Thain declined to discuss individual bonuses during the deposition so it decided on court action.

Two weeks ago, Cuomo accused Merrill Lynch of secretly accelerating bonus payments last year and giving at least $1 million to each of nearly 700 employees, even as the brokerage was amassing billions of dollars in losses.

Last week, Cuomo subpoenaed Bank of America Chairman and Chief Executive Kenneth Lewis in its investigation of whether the bank broke state law by withholding information from investors.