Foreign ministry spokesman Liu Weimin made the comment at a regular press briefing on Monday, the 23rd anniversary of the 1989 crackdown on demonstrators in which hundreds, possibly thousands, were killed.

More than two decades later, Beijing still considers the incident a "counter revolutionary rebellion" and has refused to acknowledge any wrongdoing or consider compensation for those killed.

The US State Department on Sunday called on Beijing to release those still serving sentences for their participation in the demonstrations and do more to protect the human rights of its citizens.

The call, on the eve of the highly sensitive anniversary, came amid heightened tensions between China and the United States after a major diplomatic row between over the blind Chinese dissident Chen Guangcheng.

China does not say how many people remain in custody over the 1989 protests, but the US-based rights group Dui Hua said last week it believed there were fewer than a dozen.

Any mention of the 1989 protests is banned in Chinese state media, and the subject is largely taboo in China.

The Tiananmen Mothers, a group of relatives of victims of the 1989 crackdown, issued an annual open letter to the government calling for the end of communist rule and a reassessment of the official verdict on the protests.

"So long as the Tiananmen Mothers exist, our struggle for justice will not cease," said the letter, signed by 121 members.

State Department deputy spokesman Mark Toner said Sunday the United States called on Beijing to do more to protect the human rights of its citizens -- a comment that could touch a raw nerve in Beijing on the sensitive anniversary.

The United States "joins the international community in remembering the tragic loss of innocent lives" in the "violent suppression" of the mass pro-democracy protests in the heart of Beijing, Toner said in a statement.

"We encourage the Chinese government to release all those still serving sentences for their participation in the demonstrations; to provide a full public accounting of those killed, detained or missing; and to end the continued harassment of demonstration participants and their families."

Hundreds, perhaps thousands, are believed to have died when the government sent in tanks and soldiers to clear Tiananmen Square on the night of June 3-4, 1989, violently crushing six weeks of pro-democracy protests.

More than two decades later, Beijing still considers the incident a "counter revolutionary rebellion" and has refused to acknowledge any wrongdoing or consider compensation for those killed.

The US statement made no specific mention of accounts from rights campaigners that Chinese police beat and detained political activists Sunday as they marked the anniversary of the crackdown by the People's Liberation Army.

Officers used violence against activists in the southeast province of Fujian and detained them, while more than 30 people who came to Beijing "to petition" were held and then forced to return home, the campaigners reported.

But on the heels of a major diplomatic row over blind Chinese dissident Chen Guangcheng, who is now living in the United States after fleeing house arrest, Washington urged Beijing to stop harassing human rights activists.

"We renew our call for China to protect the universal human rights of all its citizens; release those who have been wrongfully detained, prosecuted, incarcerated, forcibly disappeared, or placed under house arrest; and end the ongoing harassment of human rights activists and their families," Toner said.

In Beijing, veteran dissident Hu Jia said on his microblog that, as in previous years on the Tiananmen anniversary, police had stepped up security around the homes of numerous political activists and social critics.

Rights activists and lawyers said police had also contacted them and warned against participating in activities marking the crackdown.

Any mention of the 1989 protests is banned in Chinese state media, and the subject is largely taboo in China.

The overseas dissident website www.molihua.org had in recent days urged those opposed to the crackdown to dress in black and "stroll" in public places throughout China on June 3-4.

The call, which spread via numerous microblogs, was similar to posts last year urging Chinese to hold protests akin to those that spread through the Arab world.

The Tiananmen Mothers, a group of relatives of victims of the 1989 crackdown, issued an annual open letter to the government calling for the end of communist rule and a reassessment of the official verdict on the protests.

"So long as the Tiananmen Mothers exist, our struggle for justice will not cease," said the letter, signed by 121 members.

The only open commemoration of the crackdown to be allowed on Chinese soil will take place in Hong Kong, a Chinese territory that enjoys freedoms not allowed in the mainland.