Criminals working with computer hackers pose a rising threat to businesses and governments as those entities increase their dependence on IT systems, a top US federal prosecutor has said.

"There's always this worry, and I think a legitimate worry, that terrorists will actually launch computer attacks on critical infrastructure," said Christopher Painter, principal deputy chief of the US Department of Justice's Computer Crime and Intellectual Property Section.

Painter, who prosecuted renowned hacker Kevin Mitnick before he signed a plea agreement, cited a case where a hacker in Australia fouled a fresh water supply in 2000 by hacking into a wireless connection and releasing tons of sewage.

In the US in 1997, a juvenile hacked into a computer phone and data system and turned off the runway lights at an airport in Worcester, Massachusetts, Painter said. "That obviously could have had pretty dire consequences," Painter said, who spoke about cybercrime at the Information Security Systems Association conference in London.

Authorities have seen an increase in cases where hackers steal corporate data and then attempt to blackmail companies, Painter said. Others steal data and sell it to other criminals. In October 2004, the US Secret Service busted the "Shadowcrew", a criminal group that stole credit card data and then sold it on a website, Painter said.

The problem for law enforcement is that the bad guys are showing increased technical sophistication and often commit cybercrimes across borders. But countries are co-ordinating more on investigations and takedowns, Painter said. Forty-five countries have joined the G8 24/7 High Tech Crime Network, a group of contacts that can offer immediate assistance to other countries on terrorist or criminal cases involving electronic evidence, Painter said.

He cited a success this week where Moroccan authorities sentenced two people to prison for the Zotob and Mytob worms, which downed computers worldwide in August 2005. The US, Turkey, Morocco and Microsoft aided in that investigation, he said.

But a difficulty with international investigations are countries with weak or no laws against computer crime, Painter said. "Any country that doesn't have good laws is going to be a weak link," he said.