More cyber attacks in retaliation for attempts to block the WikiLeaks website are likely in a data war to protect Internet freedom, a representative of one of the groups involved said on Thursday.

Though no major websites appeared to be under attack early on Thursday, Swedish newspaper Aftonbladet said the Swedish government's website was down for a short time during the night. Sweden had issued an arrest warrant for WikiLeaks' founder.

Anonymous has targeted mainly companies which have decided for whatever reason not to deal with WikiLeaks. Some of the main targets involve Amazon, MasterCard, Visa and PayPal, a spokesman calling himself Coldblood told BBC Radio 4.

The campaign is not over from what I've seen, it's still going strong. More people are joining, more and more people are downloading the voluntary botnet tool which allows people to command dos (distributed denial of service) attacks, he added.

The speaker, who had an English accent, said he was aged 22 and was a software engineer.

Credit card giants MasterCard and Visa came under intense cyber attack on Wednesday as supporters of WikiLeaks retaliated for moves against WikiLeaks founder Julian Assange after the release of U.S. diplomatic cables that angered and embarrassed the Obama administration.

I see this as becoming a war, but not your conventional war. This is a war of data. We are trying to keep the Internet free and open for everyone, just the way the Internet always has been, Coldblood added.

Assange is in a London jail after he was arrested over allegations of sexual assault in Sweden. His supporters say the accusations against him are politically motivated.

It's very hard to get hold of anyone from WikiLeaks. The only (person) you could really get hold of was Julian, but unfortunately he's not available at the moment, said Coldblood.

(Reporting by Keith Weir with additional reporting by Patrick Lannin in Stockhol; editing by Mark Heinrich)