The whistleblowing group WikiLeaks claims that it has had its funding blocked and that it is the victim of financial warfare by the US government.

Moneybookers, a British-registered internet payment company that collects WikiLeaks donations, emailed the organisation to say it had closed down its account because it had been put on an official US watchlist and on an Australian government blacklist.

The apparent blacklisting came a few days after the Pentagon publicly expressed its anger at WikiLeaks and its founder, Australian citizen Julian Assange, for obtaining thousands of classified military documents about the war in Afghanistan, in one of the US army's biggest leaks of information. The documents caused a sensation when they were made available to the Guardian, the New York Times and German magazine Der Spiegel, revealing hitherto unreported civilian casualties.

WikiLeaks defied Pentagon calls to return the war logs and destroy all copies. Instead, it has been reported that it intends to release an even larger cache of military documents, disclosing other abuses in Iraq.

Moneybookers moved against WikiLeaks on 13 August, according to the correspondence, less than a week after the Pentagon made public threats of reprisals against the organisation. Moneybookers wrote to Assange: "Following an audit of your account by our security department, we must advise that your account has been closed … to comply with money laundering or other investigations conducted by government authorities."

When Assange emailed to ask what the problem was, he says he was told in response by Daniel Stromberg, the Moneybookers e-commerce manager for the Nordic region: "When I did my regular overview of my customers, I noticed that something was wrong with your account and I emailed our risk and legal department to solve this issue.

"Below I have copied the answer I received from them: 'Hi Daniel, you can inform him that initially his account was suspended due to being accessed from a blacklisted IP address. However, following recent publicity and the subsequently addition of the WikiLeaks entity to blacklists in Australia and watchlists in the USA, we have terminated the business relationship.'"

Assange said: "This is likely to cause a huge backlash against Moneybookers. Craven behaviour in relation to the US government is unlikely to be seen sympathetically."

Moneybookers, which is registered in the UK but controlled by the Bahrain-based group Investcorp, would not make anyone available to explain the decision. Its public relations firm, 77PR, said: "We have never had any request, inquiry or correspondence from any authority regarding this former customer." Asked how this could be reconciled with the references in the correspondence to a blacklist, it said: "We stick with our original statement."