London, England (CNN) -- Human Rights Watch on Saturday urged the Iraqi and U.S. governments to launch investigations into reports of torture and detainee abuse after the WikiLeaks website published thousands of classified military documents detailing the war in Iraq.
The release includes evidence that Iraqi security forces tortured and killed prisoners, the group said. Human Rights Watch called on the Iraqi government to prosecute those responsible.
It also urged the U.S. government to look into whether its forces broke international law by transferring thousands of detainees to Iraqi custody despite what Human Rights Watch called "the clear risk of torture."
"These new disclosures show torture at the hands of Iraqi security forces is rampant and goes completely unpunished," said Joe Stork, deputy Middle East director at Human Rights Watch. "It's clear that U.S. authorities knew of systematic abuse by Iraqi troops, but they handed thousands of detainees over anyway."
Also Saturday, anti-war activists said at a news conference that the WikiLeaks release revealed that 15,000 more Iraqi civilians died during the conflict than previously thought.
"We have seen that there are approximately 15,000 never previously documented or known cases of civilians who have been killed by violence in Iraq," WikiLeaks editor-in-chief Julian Assange said.
Those cases raise the civilian death toll in Iraq to 122,000, according to Iraq Body Count, an-anti-war group.
Assange said the massive leak aims to reveal hidden truths about the Iraq war.
"The attack on the truth by war begins long before war starts and continues long after a war ends," Assange said.
"In our release of these 400,000 documents about the Iraq war, the intimate detail of that war from the U.S. perspective, we hope to correct some of that attack on the truth that occurred before the war, during the war and which has continued on since the war officially concluded."
CNN was offered access to the documents in advance of the release but declined because of conditions that were attached to accepting the material. The New York Times and The Guardian, the British newspaper, were among a handful of organizations provided early access to the papers.
The documents offer a new picture of how many Iraqi civilians have been killed and show that the vast majority of slain civilians were killed by other Iraqis.
However, the Times said the documents disclosed "many previously unreported instances in which American soldiers killed civilians -- at checkpoints, from helicopters, in operations," and it said there were at least four cases of lethal shootings from helicopters.
The Guardian said the documents detail torture, summary executions and war crimes. U.S. authorities failed to investigate hundreds of reports of abuse, torture, rape and murder by Iraqi police and soldiers, the documents show, it said.
The Times said that hundreds of reports of beatings, burnings and lashings suggested that "such treatment was not an exception." Most abuse cases contained in the new batch of leaks appear to have been ultimately ignored, the paper said.
Military rules require forces to report abuse to Iraqi authorities, but suggested that there was little follow-up on abuse reports, the Times reported.
The Pentagon denounced the WikiLeaks release, and its spokesman, Geoff Morrell, rebutted allegations that the new round of field reports showed evidence of war crimes.
"We vetted every single one of the documents, word by word, page by page," Morrell told CNN, saying the process began in July. "There is nothing in here which would indicate war crimes. If there were, we would have investigated it a long time ago."
The British defense ministry Saturday condemned the "unauthorized release of classified material" that could endanger service personnel at risk. The ministry said it regrets and takes civilian casualties seriously.
"There is no place for mistreatment of detainees, and we investigate any allegation made against our troops," it said.
The documents also detail Iran's role in Iraq, including the supplying Iraqi militia fighters with weapons, including the most lethal type of roadside bomb, according to the Times.
Iraqi militants traveled to Iran for training as snipers and in using explosives. Iran's Quds Force urged Iraqi extremists it was working with to kill Iraqi officials. One document appears to lend credence to claims that Iran crossed the Iraqi border to arrest three American hikers.
Mahmoud Othman, a Kurdish lawmaker in Iraq, said he wasn't surprised when he heard about the documents.
"We also know that U.S. troops in Iraq turned a blind eye on the abuse and torture of Iraqi prisoners in the past few years," he said. "We also know about Iran's interference in Iraq and their funding of and arming of Shiite militias."
Ahmed Masari, a Sunni lawmaker from al-Iraqiya -- a mainly secular bloc with significant Sunni Arab backing -- said the reports about the documents, which he hasn't reviewed or seen, conjures agony his fellow citizens lived through.
"All the incidents listed in these documents -- we lived through this -- like the documents that mention the spontaneous shooting incidents by the Americans, coldblooded killing by the American and Iraqi forces, torture, the secret prisons," he said. "Iraqis have suffered a lot, and our country can't bear anymore agony."
Iraqi Prime Minister Nuri al-Maliki's office said it carries out arrests legally and doesn't target people on a "sectarian or partisan basis -- as some factions are trying to say for some reasons."
WikiLeaks called the release the largest classified military leak in history. After the leak in July of more than 70,000 Afghanistan war documents, the website was heavily criticized by the U.S. government, the military and human rights groups for failing to redact names of civilians in the documents, putting them at risk of retaliation by the Taliban.
Assange said WikiLeaks has been more "vigorous" in redacting the new round of documents compared with its previous publication of documents. But the U.S. military said about 300 Iraqis named in the documents could be endangered by their exposure and they are being notified for their safety.
In a statement about the documents, Amnesty International urged the United States to probe how much American officials knew about the torture and abuse of detainees held by Iraqi security forces.
"We have not yet had an opportunity to study the leaked files in detail, but they add to our concern that the U.S. authorities committed a serious breach of international law when they summarily handed over thousands of detainees to Iraqi security forces who, they knew, were continuing to torture and abuse detainees on a truly shocking scale," said Malcolm Smart, Amnesty International's director for the Middle East and North Africa.
John Sloboda, the Iraq Body Count's co-founder, told reporters that the names of civilian victims are among the details included in the documents.
"Almost every log tells a story, and far too often, this is a previously unknown story of human suffering and death," he said.
Sloboda said the meticulous records kept by the U.S. military and published by WikiLeaks will be a valuable tool in investigating civilian casualties in the Iraq war.
Phil Shiner of the U.K.-based Public Interest Lawyers, a firm specializing in international and human rights law, told reporters that some information in the documents would be the subject of legal action in the United Kingdom.
He alleged the documents revealed details about unlawful killings of civilians, indiscriminate attacks against them and unjustified use of lethal force.
"There must now be a judicial inquiry into all these deaths," he said.
Pentagon Papers whistle-blower Daniel Ellsberg, responsible for leaking the U.S. government's top secret study on the Vietnam War in 1971, attended the press conference and praised Assange.
"I want very much to congratulate all of you who are mining this material to learn what we could have learned if it had come out earlier," Ellsberg said.