American officials are searching for Julian Assange, the founder of WikiLeaks in an attempt to pressure him not to publish thousands of confidential and potentially hugely embarrassing diplomatic cables that offer unfiltered assessments of Middle East governments and leaders.

The Daily Beast, a US news reporting and opinion website, reported that Pentagon investigators are trying to track down Julian Assange – an Australian citizen who moves frequently between countries – after the arrest of a US soldier last week who is alleged to have given the whistleblower website a classified video of American troops killing civilians in Baghdad.

The soldier, Bradley Manning, also claimed to have given WikiLeaks 260,000 pages of confidential diplomatic cables and intelligence assessments.

The US authorities fear their release could "do serious damage to national security", said the Daily Beast, which is published by Tina Brown, former editor of Vanity Fair and New Yorker magazines.

Manning, 22, was arrested in Iraq last month after he was turned over to US authorities by a former hacker, Adrian Lamo, to whom he boasted of leaking the video and documents.

As an intelligence specialist in the US army, Manning had access to assessments from the battlefields in Afghanistan and Iraq, as well as frank diplomatic insights into Middle East governments.

In one of his messages to Lamo, obtained by Wired magazine, Manning said: "Hillary Clinton and several thousand diplomats around the world are going to have a heart attack when they wake up one morning and find an entire repository of classified foreign policy is available."

Although it is likely that WikiLeaks has broken US laws in de-encrypting the video from Baghdad and publishing secret documents, the tone of an American official who spoke to the Daily Beast sounded more desperate than threatening. "We'd like to know where he is; we'd like his cooperation in this," the official said.

It is, in any case, not clear what legal measures US officials could use to stop publication of the cables. Assange has created an elaborate web of protection – with servers in several countries, notably Sweden, which has strong laws protecting whisteblowers.

WikiLeaks' response to the news that the Americans are trying to track down Assange came on Twitter. "Any signs of unacceptable behaviour by the Pentagon or its agents towards this press will be viewed dimly," it said.

After Manning was arrested, WikiLeaks said in a Twitter message that allegations "we have been sent 260,000 classified US embassy cables are, as far as we can tell, incorrect".

Before his arrest, Manning told Lamo he was in part motivated to leak the video and documents by being ordered to look the other way in the face of injustice.

Messages from Manning, obtained by Wired, say he found that 15 Iraqis arrested by Iraqi police for printing "anti-Iraq" literature had merely put together an assessment of government corruption.

"I immediately took that information and ran to the [US army] officer to explain what was going on. He didn't want to hear any of it. He told me to shut up and explain how we could assist the [Iraqi police] in finding MORE detainees," Manning wrote.

"Everything started slipping after that. I saw things differently. I had always questioned the [way] things worked, and investigated to find the truth.

"But that was a point where I was … actively involved in something I was completely against."

The Pentagon has declined to comment on the grounds that what is in the documents is classified.