Kabul, Afghanistan (CNN) -- Afghanistan's president said he is incensed that leaked secret U.S. military documents include the names of Afghan informants.
"This indeed is very irresponsible and shocking," President Hamid Karzai told reporters on Thursday.
"Because whether those individuals acted legitimately or illegitimately in providing information to the NATO forces, they are lives, and their lives will be in danger now."
U.S. military officials were assessing what damage could be done to intelligence contacts in Afghanistan after a number of names of local Afghans working with the U.S. military appeared on documents leaked by the WikiLeaks website. Officials are concerned that potential new contacts may be reluctant to help out of the fear that the United States can't protect sources.
Karzai said that authorities must determine the context in which those names are mentioned "and then act accordingly."
"This is a very serious issue," he said.
The Pentagon is focusing on jailed Army Pfc. Bradley Manning as the main suspect in the leak of tens of thousands of secret U.S. military documents related to the war in Afghanistan, a senior Pentagon official told CNN Wednesday.
Manning, 22, is believed to have accessed a worldwide military classified Internet and e-mail system to download the documents, according to the official, who did not want to be identified because of the ongoing criminal investigation.
The Pentagon official said investigators now believe Manning logged into a system called the Secret Internet Protocol Router Network, which essentially provides military members who have appropriate security clearances access to classified e-mails and the military's classified Internet system.
Pentagon officials have said for the past several days that so far the only material they have seen on the website WikiLeaks.org is classified at the "secret" level, a relatively low-level designation that allows for a large number of military personnel to access the information.
Julian Assange, WikiLeaks founder, has refused to say where his whistle-blower website got about 91,000 United States documents about the war. Some 76,000 of them were posted on the site Sunday in what has been called the biggest leak since the Pentagon Papers about the Vietnam War.
The Pentagon has said it has not seen any top-secret information in any of the papers, and both the Defense Department and the White House say they don't contain much new information about the war, now nearly nine years old.
Manning was charged in June with eight violations of the U.S. Criminal Code for allegedly illegally transferring classified data, reportedly including an earlier video that wound up on WikiLeaks.org. The private had top-secret clearance as an intelligence analyst for the Army when he was stationed in Iraq.
Adrian Lamo, a self-described California hacker, said he doesn't regret going to authorities about a person he believes confessed to him in an online chat about leaking thousands of sensitive documents about the Afghanistan war.
Lamo's alleged chats with a person believed to be Manning have been published in Wired magazine. CNN asked Lamo for a copy of the entire chat Wednesday night but he declined to provide it because he said it contained personal information and information that might compromise national security. He said he turned over his hard drive to the Department of Defense.
"It seemed incomprehensible that someone could leak that massive amount of data and not have it endanger human life," Lamo told CNN in a phone interview late Wednesday night. "It boils down to that. If I had acted for my own comfort and convenience and sat on my hands with that information, and I had endangered national security ...
"We think of national security as this linguistic buzz word, but it's more human than that," Lamo said. "National security is about individuals. At the end of the day, it's about the people who guard our national security and the people who benefit from it. With that in mind, I felt I would be the word kind of coward if I had simply sat by."
The documents released by WikiLeaks are divided into more than 100 categories and touch on everything from the hunt for al Qaeda leader Osama bin Laden to Afghan civilian deaths resulting from U.S. military actions. Thousands of pages of reports document attacks on U.S. troops and their responses, relations between Americans in the field and their Afghan allies, intramural squabbles among Afghan civilians and security forces and concerns about neighboring Pakistan's ties to the Taliban.
CNN has not independently confirmed the authenticity of the documents, but neither the White House nor the Pentagon has denied that they are what WikiLeaks claims they are.
At his press conference on Thursday, Karzai said Afghan officials are hopeful that allies and neighboring nations "are learning something new from these documents and we learn lessons for our future strategy."
In an apparent reference to allegations in the documents that Pakistan's spy service was helping militants in Afghanistan, Karzai said Afghans have long known that secret services of foreign countries were influencing Afghanistan. But he said the WikiLeaks documents are something new for the international community, which could learn lessons to help them formulate strategies.
"Afghans have known what has been going on in their country and the Afghans were raising their voices in this regard, but this is something else as to why the world was not paying attention to it," he said.
CNN's Matiullah Mati and Ashley Fantz contributed to this report