Julian Assange, a renowned Australian hacker who founded the electronic whistleblowers' platform WikiLeaks, vanished when a young US intelligence analyst in Baghdad was arrested.

The analyst, Bradley Manning, had bragged he had sent 260,000 incendiary US state department cables on the wars in Iraq and Afghanistan to WikiLeaks.

The prospect of the cache of classified intelligence on the US conduct of the two wars being put online is a nightmare for Washington. The sensitivity of the information has generated media reports that Assange is the target of a US manhunt.

"[US] public statements have all been reasonable. But some statements made in private are a bit more questionable," Assange told the Guardian in Brussels. "Politically it would be a great error for them to act. I feel perfectly safe … but I have been advised by my lawyers not to travel to the US during this period."

Assange appeared in public in Brussels for the first time in almost a month to speak at a seminar on freedom of information at the European parliament.

He said: "We need support and protection. We have that. More is always helpful. But we believe that the situation is stable and under control. There's no need to be worried. There's a need always to be on the alert."

Manning is being held incommunicado by the US military in Kuwait after "confessing" to a Californian hacker on a chatline, declaring he wanted "people to see the truth".

He said he had collected 260,000 top secret US cables in Baghdad and sent them to WikiLeaks, whose server operates out of Sweden. Adrian Lamo, the California hacker he spoke to, handed the transcripts of the exchanges to the FBI.

Manning was promptly arrested in Baghdad at the end of last month and transferred to a US military detention unit in Kuwait. He has been held for more than three weeks without charge.

Assange said WikiLeaks had hired three US criminal lawyers to defend Manning but that they had been granted no access to him. Manning has instead been assigned US military counsel.

While WikiLeaks declined to confirm receipt of the material from Manning, it has already released a film of a US Apache helicopter attack on civilians in Baghdad.

It has also posted a confidential state department cable on negotiations in Reykjavik over Iceland's financial collapse and is preparing to disclose much more material, including film of a US attack that left scores of civilians dead in Afghanistan.

The material is believed to derive from Manning, although WikiLeaks does not reveal its sources and its operations are designed to mask the source of the files it receives.

Prominent US whistleblowers and lawyers have advised Assange to stay out of the US and to be ultra-careful about his travel and public appearances. "Pentagon investigators are trying to determine the whereabouts of [Assange] for fear that he may be about to publish a huge cache of classified state department cables that, if made public, could do serious damage to national security," US web paper the Daily Beast reported 10 days ago.

"We'd like to know where he is – we'd like his co-operation in this," a US official was quoted as saying.

Daniel Ellsberg, who leaked the Pentagon Papers – a top secret study about the Vietnam war – in 1973, spoke to the Daily Beast.

He said: "I would think that [Assange] is in some danger. Granted, I would think that his notoriety now would provide him some degree of protection."

Assange said: "Some fear for my life. I'm not one of them. We have to avoid some countries, avoid travel, until we know where the political arrow is pointing."

He added that WikiLeaks had been trying, "unsuccessfully so far", to contact Manning in Kuwait.

"Clearly, a young man is detained in very difficult circumstances with the allegation he is the whistleblower. We must do our best to obtain freedom for him."

Regarding his own predicament, Assange said the US state department had signalled it was not seeking any WikiLeaks people because the Pentagon's criminal investigations command had assumed the lead role in the case.

Apart from preparing much more material for release, WikiLeaks is planning to publicise a secret US military video of one of its deadliest air strikes in Afghanistan in which scores of children are believed to have been killed in May last year.

The Afghan government said about 140 civilians were killed in Garani, including 92 children. The US military initially said that up to 95 died, of whom about 65 were insurgents.

US officials have since wavered on that claim. A subsequent investigation admitted mistakes were made.

In April WikiLeaks released the Baghdad video, prompting considerable criticism of the Pentagon.

The film was edited and produced in Iceland where Assange spends a lot of his time and which last week prepared the most radical and liberal freedom of information legislation anywhere in the world.

Birgitta Jonsdottir, an Iceland MP and anti-war activist who led the drive for the new laws, co-produced the WikiLeaks version of the Baghdad video.

"I worked on it 18 hours a day through the Easter holidays," she said.

Jonsdottir, a close associate of Assange, said the WikiLeaks founder "went into hiding when the story of Manning's arrest was published".