Japan's operation of military brothels during World War II was "deplorable," but the issue can only be resolved by Tokyo and its Asian neighbors, a top U.S. official said Friday, as victims expressed outrage at the Japanese prime minister's denial that women were forced into sexual slavery.

Deputy Secretary of State John Negroponte spoke a day after Japanese Prime Minister Shinzo Abe denied the women were forced into prostitution, calling into doubt an apology by a top government spokesman in 1993.

Negroponte refused to comment directly about Abe's words, but he said he hoped disagreements between Japan and its neighbors would not interfere with regional cooperation on other important issues.

"Our view is that what happened during the war was most deplorable," he said when asked about the sex slave issue. "But ... as far as some kind of resolution of this issue, this is something that must be dealt with between Japan and the countries that were affected."

Up to 200,000 women served in Japanese frontline brothels known as "comfort stations" during the war.

"The fact is, there is no evidence to prove there was coercion," Abe said Thursday.

His remarks contradicted evidence in Japanese documents unearthed in 1992 that historians said showed military authorities had a direct role in working with contractors to forcibly procure women for the brothels.

Women's rights activists in the Philippines denounced Abe's comment.

"We are enraged," said Rechilda Extremadura, executive director of Lila Pilipina, an organization of activists and former Filipino wartime sex slaves.

Thousands of women from the Philippines and other Japanese-occupied nations were forced to work in brothels run by Japan's military during WWII.

"We will not allow them to deny it just like that," Extremadura said. "For us, good or bad, it is your history. If you are a responsible government, you are responsible enough to accept, acknowledge and be accountable."

Witnesses, victims and even some former Japanese soldiers say many of the women -- most of them from Korea and China -- were kidnapped or otherwise forced into sexual slavery at the brothels, where they could be raped by scores of soldiers a day.

Victims and their supporters have pushed unsuccessfully for a parliament-approved apology from Japan and official government compensation. Tokyo set up a private fund for compensation in 1995, but has refused to provide government money.

The United States has avoided public involvement in historical disputes between Japan and its neighbors, though it has expressed concern that such conflicts could affect other issues, such as cooperation on efforts to get North Korea to abandon its nuclear weapons program.

Last month, however, the House of Representatives held hearings on a resolution calling for Japan to fully acknowledge and apologize for the sexual abuse. Abe, however, told reporters on Thursday that "there was no evidence to prove there was coercion" in the running of the brothels.

Hilaria Bustamante, an 81-year-old member of Lila Pilipina, said she was a sex slave in a Japanese garrison for more than a year.

Recounting her horror, she said she was heading home in 1942 after scavenging for rice grains at a farm when three Japanese soldiers stopped her on the road and seized her by the arms and legs and threw her into a truck "like a pig."

"Even as I struggled, I could not do anything. They slapped me, they punched me. I was only 16 then, what could I do?" she said. "They think we are like toilet paper that is just thrown after being used."