US President Barack Obama has refused to comment on Chen Guangcheng, the Chinese dissident said to be at the US embassy after fleeing house arrest.
Mr Obama told a news conference he was "aware of press reports" on the issue, but would not make a statement on it.
Activists have claimed Mr Chen entered the US embassy in Beijing earlier this month, after slipping out of his home hundreds of kilometres away.
US and Chinese officials are thought to be in talks on Mr Chen's fate.
State Department spokesman Victoria Nuland confirmed that a senior US official, Kurt Campbell, has travelled to Beijing, but declined to say whether he was in discussion about the activist.
Mr Campbell, who arrived on an unscheduled visit, is believed to be in highly delicate negotiations with the Chinese authorities about what to do with the blind activist.
The BBC's Adam Brookes in Washington says one of the options on the table is for Mr Chen to go into exile, but the activist is said to be averse to that solution.
Ms Nuland said Mr Campbell's visit was connected with preparations for a visit by US Secretary of State Hillary Clinton on Wednesday.
Ms Clinton said on Monday that she would raise the issue of human rights during her talks in China.
"A constructive relationship includes talking very frankly about those areas where we do not agree, including human rights," she was quoted saying by the Reuters news agency.
Mr Chen was placed under house arrest in Shandong province in 2010 after spending more than four years in jail for disrupting traffic and damaging property.
He is thought to have escaped the house where he was held more than a week ago, although the news only emerged at the end of last week. Activists say he was then driven to the US embassy.
The activist, who has been blind since childhood, has long been a high-profile figure and international rights groups have frequently expressed alarm at the treatment of him and his family.
Asked to comment on Mr Chen's case, Mr Obama said he could only stress that "every time we meet with China the issue of human rights comes up".
"We want China to be strong, we want it to be prosperous and we are very pleased with all the areas of co-operation that we have been able to engage in," he told a joint news conference with visiting Japanese Prime Minister Yoshihiko Noda.
"But we also believe that that relationship will be that much stronger and China will be that much more prosperous and strong as you see improvements on human rights issues in that country."
US officials have often raised Mr Chen's case with China on human rights grounds.
Earlier, the EU urged China to "exercise utmost restraint" in connection with Mr Chen, amid reports that some close to the activist had been rounded up.
Mr Chen exposed how local authorities in Linyi, in Shandong province, forced thousands of women to have abortions or be sterilised as part of China's one-child policy.
His colleagues said last Sunday's escape had taken months to plan, and was carried out with the help of a network of friends and activists.
He scaled the wall that the authorities had built around his house, and was driven hundreds of miles to Beijing, where activists say he stayed in safe houses before fleeing to the embassy.
Several people involved in Mr Chen's escape have been detained or have disappeared in recent days, and fellow activist Hu Jia has been questioned.