A foreign ministry official told The Associated Press news agency that it was not yet known when Spain would give final approval for the transfers.
A number of other European countries have agreed to take in Guantanamo inmates, including France, Hungary, Ireland, Italy and Portugal.
Barack Obama, the US president, has pledged to close the prison camp, but has missed his January 22 deadline partly because it has not yet resolved where the remaining inmates will be sent.
Washington asked other countries to help by taking in detainees cleared of any charges who cannot be sent back to their homelands, often because of fears they could be persecuted there.
Daniel Fried, Obama's special envoy for the task, met with Spanish officials in Madrid last June and asked the country to take in four prisoners.
Jose Luis Rodriguez Zapatero, Spain's prime minister, has been eager to establish good ties with the US, after angering then-president George Bush in April 2004 by withdrawing Spanish peacekeepers from Iraq.
Zapatero has also agreed to send more Spanish troops to Afghanistan in response to a plea from Obama for more allied help in fighting the Taliban.