HONG KONG — Chinese authorities have begun blocking access from mainland China to the Web site of The New York Times even while lifting some of the restrictions they had recently imposed on the Web sites of other media outlets.

When computer users in cities like Beijing, Shanghai and Guangzhou tried to connect on Friday morning to nytimes.com, they received a message that the site was not available; some users were cut off on Thursday as early as 8 p.m. The blocking was still in effect on Saturday morning.

But the Chinese-language Web sites of BBC, Voice of America and Asiaweek, all of which had been blocked earlier this week, were accessible by Friday. The Web site of Ming Pao, a Hong Kong newspaper, was blocked earlier this week and still restricted on Friday.

Chinese officials had few explanations for the restriction on The Times’s site. “Concerning your particular question, we’re not really familiar with the details,” said a spokesman for the Ministry of Foreign Affairs in Beijing, who declined to give his name. “Web site maintenance is not within the job purview of the Foreign Ministry.”

Tang Rui, an official with the government’s International Press Center in Beijing, said he also had no specific information. “It might be a technical problem,” he said, declining to elaborate.

Access to the Web site was not restricted on Friday in Hong Kong, which Britain returned to Chinese rule in 1997 but which still allows freedom of speech, including on the Internet. Internet users in Japan and the United States were also not experiencing difficulties on Friday in viewing the site.

A spokeswoman for The Times, Catherine J. Mathis, said there did not appear to be a technical issue.

Rebecca MacKinnon, a researcher at Hong Kong University who specializes in China’s Internet controls, said the reasons for the restrictions were mysterious. “All anybody can offer is speculation,” she said.

In the months leading up to the Olympics in Beijing, during the Games and immediately after, the Chinese government temporarily unblocked access to some Web sites and eased curbs on the ability of foreign correspondents to travel within China. It has not tightened the travel restrictions since then