US State Department workers were horrified by WikiLeaks' publication of more than 250,000 diplomatic cables leaked by soldier Bradley Manning, an agency official testified Thursday at his sentencing.

Elizabeth Dibble didn't give any evidence in open court of how the largest leak of classified information in the country's history damaged US foreign relations, but she did testify in a session closed to the public to protect classified information.

Dibble was the No 2 official in the agency's bureau of near-eastern affairs when WikiLeaks began publishing the cables on its website in late 2010.

Manning was convicted earlier this week. The sentencing hearing is to determine how long he will be in prison for leaking the cables, plus more than 470,000 Iraq and Afghanistan battlefield reports and some battlefield video, while working as an intelligence analyst in Iraq.

Manning faces up to 136 years in prison following his conviction on 20 counts, including Espionage Act violations, theft and computer fraud. He has said he leaked the material to expose wrongdoing by the military and US diplomats.

Dibble said people in her office reacted with "horror and disbelief that our diplomatic communications had been released and were revealed on public websites for the world to see."

She was called to testify specifically about the impact of the leaks in Iran, Lebanon and Libya.

A May 2011 report by the State Department's inspector general's office said the WikiLeaks disclosures contained more information from embassies in Dibble's region than any other. The impact on US relations with those countries was immediate and, in some cases, profound, the report said.

"The US ambassador in Tripoli was quickly recalled after a particularly strong reaction from the Libyan government," the report said.

Then secretary of state Hillary Clinton said in November 2010 that the leaks endangered people's lives, threatened national security and undermined US diplomatic efforts.

Manning's lawyers maintain the leaks did little or no harm.

Dibble said she disagreed with the sentiments expressed by then-defense secretary Robert Gates in December 2010: "I've heard the impact of these releases on our foreign policy described as a meltdown, as a game-changer and so on. I think those descriptions are fairly significantly overwrought."