US “deeply concerned” at detention of Chinese dissident Ai Wei Wei

The U.S. is "deeply concerned" about disappearances and arrests of human rights activists in China, according to a spokesman responding to the arrest of artist Ai Wei Wei.

The arrest has fueled speculation that Beijing's ongoing crackdown to prevent protests similar to those seen in the Middle East and North Africa is reaching a more aggressive phase.

Dozens of Chinese lawyers and activists have vanished or been arrested since February, when calls were made by online activists for protests similar to the pro-democracy uprisings that have ousted the leaders of Egypt and Tunisia and caused others to crack down hard on their opponents.

No major public protests have taken place in China, although several websites have called for a series of weekly "Jasmine Rallies" across China during recent Sundays to press for a more accountable government. The anonymous organizers, believed to be overseas Chinese dissidents, posted their calls for protest on the Chinese-language Boxun.com and other websites.

Ai, 53, who helped design the futuristic Bird's Nest stadium at the Beijing Olympics, has been missing since Sunday, just before he was due to fly to Hong Kong, according to the Associated Press. Police also raided his Beijing studio.

His assistant Jennifer Ng told NBC News that police had been “civil” at the airport on Sunday when Ai — an outspoken critic of the government who has more than 70,000 followers on Twitter — had been blocked from boarding a flight to Hong Kong, telling her that Ai had “other business” and could not take the flight.

Ai's lawyer, Liu Xiaoyuan, late on Monday criticized the Chinese media on Twitter for not reporting the artist's disappearance. "Ai Wei Wei has been taken away, and his studio has been searched," he wrote. "Whether Ai Wei Wei is right or wrong, this is still really big news, a really hot topic. I never thought, never thought, that the domestic media would actually lose the power of speech, and act both deaf and dumb. Sad, really sad."

Police also took in Ai's wife, Lu Qing, and eight of his assistants for questioning Sunday, but released them late Sunday night following questioning, according to the Guardian. However, Ai and a friend were still missing more than 36 hours after being detained.

France and Germany are leading calls for Ai's release.  "I appeal to the Chinese government to urgently provide clarification, and I expect Ai Wei Wei to be released immediately," the German foreign minister, Guido Westerwelle, said in a statement, the Guardian reported. Ai has reportedly said that he is building a studio in Berlin, in response to the increasing pressure he faced in China.

In Washington, State Department spokesman Mark Toner called for the immediate release of Ai.

"We obviously continue to be deeply concerned by the trend of forced disappearances, extralegal detentions, arrests and convictions of human rights activists for exercising their internationally recognized human right for freedom of expression," Toner told a news conference, the AP reported. "Our relationship with China is very broad and complex but it's an issue where we disagree and we continue to make clear those concerns."

Chinese President Hu Jintao said in Washington in January that his country need to "do more" on human rights. Detention of rights activists has since increased.