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The FBI says it is investigating reports in the US media of recent cyber-attacks against several US banks.

The reports suggest between two and five banks have been targeted, including Wall Street giant JP Morgan Chase.

JP Morgan Chase declined to comment on the reports directly, but said companies of its size experienced cyber-attacks "nearly every day".

The FBI did not indicate who it suspected of being behind the attacks.

A statement from the bureau said it was working with the US secret services to determine the scope of the attacks.

But Bloomberg News, which first reported the attacks, said the investigation was looking at the possible involvement of Russia, amid worsening relations with the US over crises in Ukraine and the Middle East.

Bloomberg quoted security experts saying that the capabilities of the hackers to breach several layers of security appeared to be "far beyond the capability of ordinary criminal hackers".

Eastern Europe

But others questioned why this attack involved the apparent theft of data, rather than the disruption of services more characteristic of retaliatory attacks from state actors.

"This is very different from the alleged Iranian attacks earlier in 2012 and late 2013 that were purely of a denial-of-service nature," said Amichai Shulman, chief technology officer at security firm Imperva.

"I find it odd that someone who was actually able to break into a bank is not using it for making immediate profit.

"Everyone is trying hard to tie this with the whole political situation with Russia. However, it is well known that for a few years now, a large portion of banking attacks and financially related hacking has consistently been coming from Eastern Europe."

A spokeswoman for JP Morgan Chase said: "Companies of our size unfortunately experience cyber-attacks nearly every day.

"We have multiple layers of defence to counteract any threats and constantly monitor fraud levels."