The United States will only allow British detainees at Guantanamo Bay (searchto return home if they are prevented from engaging in terrorist activity, an American official was quoted as saying Friday.

Pierre-Richard Prosper (search), the U.S. ambassador at large for war crime issues, said the nine Britons being held at the naval base in Cuba posed a serious or medium threat, The Times of London newspaper reported.

Prosper told the newspaper that the detainees would have to be "detained and investigated, and/or prosecuted" if they came back to Britain.

"There can't be a situation where a dangerous person is released and (flies) an airplane into the next tall building around the world. That concern remains," Prosper was quoted as saying.

"We are not asking for absolutes. We are not asking for a guaranteed conviction," he said. "But we are saying: these are dangerous people, they are engaged in dangerous activity."

Prime Minister Tony Blair (searchhas said talks are under way to determine the fate of the British detainees at Guantanamo. He said Wednesday that he would make an announcement to Parliament on the issue "shortly."

The Foreign Office said it couldn't comment on The Times report, adding only that it expected Blair's announcement to be made in the next few weeks.

Prosper told The Times that negotiations for the prisoners' return were examining each case individually.

"We are asking that they be detained and investigated, and/or prosecuted," he said. "But it is not just a blanket request we have put in.

"What makes it much more complex is that we have to have these discussions on each individual."

Prosper said the Guantanamo detainees fell into three categories: Those perceived as the most serious threat, those who posed a medium threat and those who posed no threat or a low threat. He said the Britons fell into the first two categories.

The United States holds about 660 prisoners from 44 countries at the base in eastern Cuba but declines to provide a breakdown of their citizenship, ages or the reasons they are being held.

U.S. authorities have not charged them or given them access to lawyers.