Hu Jia, 31, was under house arrest and in the company of police when he was last seen, according to his wife.
Joel Rehnstrom, the country co-ordinator for the UNAIDS China office, said his agency often works with Hu and was informed last week that he disappeared on 16 February.
"We, as with many others, have received periodic communication from his wife and have, how shall I say, conveyed what we know about his situation to the Ministry of Health and expressed our concern regarding his situation," Rehnstrom said on Tuesday:
He said the ministry had not responded nor did he expect it would.
"We wouldn't normally expect a report back to us. That's not the way the government works here."
Liu Jianchao, a Chinese Foreign Ministry spokesman, would not confirm whether any detentions linked to the hunger strike had taken place.
Awareness of HIV/Aids issues
in China remains minimal
Liu said: "I don't think people in China would be arrested simply because of their thinking. If they do not violate the law they will not be arrested or detained in China."
Asked whether hunger strikes were illegal in China, Liu said that protests and demonstrations "should be conducted under a legal framework" and did not elaborate.
Hu is one of hundreds of Chinese dissidents who have staged 24-to 48-hour hunger strikes or promised to do so in support of a protest launched on 6 February by Gao Zhisheng, a activist Beijing lawyer.
The strike is meant to protest violence against dissidents, Gao says.
Rehnstrom said: "Given (Hu's) past history in antagonising government authorities or different parts of the government, it makes us worried regarding his situation."
"I don't think people in China would be arrested simply because of their thinking. If they do not violate the law they will not be arrested or detained in China"
Chinese Foreign Minsitry spokesman
In May 2004, Hu said he was placed under house arrest to prevent him from travelling to an Aids-infected village in central Henan province at the same time that an American Embassy delegation was visiting.
Hu's wife, who asked not to be identified by name, said she last saw her husband when she left for work on the morning of 16 February.
She said several plainclothes police whom she recognised were in her building lobby that morning and sitting in cars around her home.
She said she has filed a missing person report with police but has had no news.
Yu Zhijian, a former teacher who spent 11 years in jail for throwing paint at a portrait of Mao Zedong that overlooked the 1989 Tiananmen square pro-democracy protests, was detained on 18 February after his hunger strike, his girlfriend Su Gui told The Associated Press by telephone on Tuesday.
Su said Yu was taken in by police in Changsha, the capital of south China's Hunan province, and has since been moved to a detention centre in Liuyang city.
Police told her Yu is being investigated for subversion of state power.
A man who answered the phone at the Liuyang Detention Centre and who refused to give his name said no one named Yu Zhijian was in custody there.
According to the lawyer Gao, at least ten people have been detained by police since 15 February and held without charges in the cities of Beijing and Shanghai, as well as the south China provinces of Guizhou and Yunnan.
Among those still missing is Qi Zhiyong, a Beijing pro-democracy activist, who was crippled after being shot in the leg when army troops were sent in to end the Tiananmen protests, Gao said.
He said others include Liao Shuangyuan, a Guizhou activist; Zhao Xing, a Yunnan dissident, Mao Hengfeng, a Shanghai woman who has fought China's restrictive one-child policy; and Ouyang Xiaorong, Gao's office assistant.