Mr Bellinger said it would be extremely difficult to resettle detainees in the US
A senior US official has described as a "significant step" Portugal's offer of asylum for some inmates from the US detention centre at Guantanamo Bay.
John Bellinger, legal adviser to the US secretary of state, told the BBC the move was the first break in a European refusal to help shut down the camp.
In a letter to EU members this week, Portugal urged them to follow its lead.
The US has cleared 50 to 60 detainees for release, but it cannot repatriate them due to the risk of mistreatment.
President-elect Barack Obama has pledged to close down the detention centre soon after he takes office in January, but he is yet to set out what will happen to the 250 men currently being held there.
In his letter on Thursday, Portuguese Foreign Minister Luis Amado said the European Union "should send a clear signal of our willingness to help the US government resolve this problem, namely by taking in the detainees".
"The time has come for the European Union to step forward," he wrote.
Albania is the only country to have so far accepted Guantanamo detainees, taking in five members of China's Uighur ethnic minority on humanitarian grounds in 2006.
Mr Bellinger said the Portuguese government's public offer was, therefore, "really quite a significant initiative that we welcome very much".
The state department's legal adviser said that there were 50 to 60 so-called "hard cases" at Guantanamo, including several Uighurs, who the US has been unable to repatriate because of human rights concerns in the home countries.
"[The Uighurs] were properly detained, they were in training camps… but they wanted to fight the Chinese. So there's no question that we had the proper authority to detain them," he told the BBC in an interview.
"Since we determined who they were, and that they were not intent on fighting us, we've been trying to release them. But China is the only country that wants them back," he added.
Beijing has frequently cracked down on Uighur dissidents, who it accuses of seeking an independent homeland in the western Chinese province of Xinjiang.
Mr Bellinger said American immigration laws were such that it would be extremely difficult to resettle them in the US, so he welcomed the "first breaking of the ice in European resistance in trying to help out".
In October, a federal judge ordered the government to allow a group of 17 Uighurs at Guantanamo to live in the US, but their transfer has been held up by appeals.