Bangladesh has formally sought assistance from the U.S. Federal Bureau of Investigation to track down the cybercrooks who stole $81 million from its central bank's U.S. account, the interior minister said on Saturday.

Unknown hackers breached the computer systems of Bangladesh Bank in early February and attempted to steal $951 million from its account at the Federal Reserve Bank of New York, which it uses for international settlements.

Some attempted transfers were blocked, but $81 million was transferred to accounts in the Philippines in one of the largest cyberheists in history.

The central bank governor resigned on Tuesday, as details emerged in the Philippines that $30 million of the money was delivered in cash to a casino junket operator in Manila, while the rest went to two casinos.

"We sought the FBI's assistance when a group of FBI met with me for investigating the central bank heist last month," Interior Minister Asaduzzaman Khan told Reuters.

An U.S. embassy official in Dhaka told Reuters Washington stood ready to assist the government of Bangladesh in its investigation.

A senior police official involved in the investigation said that an FBI team was expected to visit the Criminal Investigation Department (CID) of police in Dhaka on Sunday. The CID was also coordinating with Interpol to track down the perpetrators.

"We are trying to find out what type of security there was, what safety measures were taken, and how the thieves penetrated the fire wall," he said.

The chief of the main opposition Bangladesh Nationalist Party said on Saturday that if elected her party would ensure real autonomy for the central bank.

"There will be no interference or influence from the political party," Begum Khaleda Zia said at a party conference.

A central bank official said two engineers from secure network operator SWIFT (the Society for Worldwide Interbank Financial Telecommunication) had started tests on Saturday.

He said the system would be upgraded if needed.

"Bangladesh is a member of the SWIFT society, so we use its service for international transactions," the official added.