"This will weaken diplomacy around the world. It will weaken diplomacy in general, but first and foremost American diplomacy. I see this rather as something that is making the world less safe," Bildt told Swedish public radio.
"There is a need for confidential communication between different governments ... to be able to handle crises and difficult situations," he said, lamenting that "we are not going to be able to have that now."
"Then we will be stuck with megaphone diplomacy, which I know from experience can lead to more conflicts and more problems," he added.
Whistleblower website WikiLeaks began on Sunday what it says will be a slow release of more than 250,000 diplomatic cables, unveiling a number of shocking diplomatic episodes, from a nuclear standoff with Pakistan to Arab leaders urging a strike on Iran.
Bildt stressed that the cables were "written by individuals," stressing the need for "diplomats to have the freedom to make fairly uninhibited evaluations ... without needing to publish a press release."
"If you undermine diplomacy in the way that is now being done, you are also undermining the possibility to hinder conflicts and promote stability," he said.
A total of 819 of the diplomatic documents yet to be released are known to concern Sweden, and the Expressen daily reported Monday it had received information the files mainly related to Sweden's European Union presidency last year.
One document, according to the paper, criticised the Scandinavian country for leading the European resistance to imposing sanctions on Iran.
Bildt did not discuss what elements may have been revealed in the Sweden-related documents, but said in his radio interview that the release of these files "of course is not good."
"It makes it much more difficult to gain confidence," he said.
The Swedish foreign minister was also critical of the US security lapses that made it possible for the sensitive documents to leak out.
"I think this is very bad," he said, adding "it will take a long time to repair the damage."
"I suspect that if this (security problem) has not already been fixed, it will be soon."
Chris Dunnett, a spokesperson for the US embassy in Stockholm welcomed Bildt's remarks about the leak, admitting that the affair was "embarrassing" for the US government.
"We deeply regret the disclosure of information from conversations that are meant to be confidential," he told The Local.
"The ability to have an honest dialogue with your partner is part of the basic bargain of diplomacy."
Dunnett doubted, however, that US-Swedish relations would be adversely affected by the leak.
"We've always had very strong relations with Sweden and I'm confident that those strong relations will continue," he said.