The International Committee of the Red Cross says it is urgently sending a doctor to Guantanamo Bay because of a growing hunger strike among detainees.
The ICRC says the doctor and another group member are flying to Guantanamo a week earlier than planned "because of the current tensions" there.
A Pentagon spokesman says 31 of 166 detainees are now on hunger strike.
The inmates - many of whom are being held without charge - are angry at the US failure to resolve their fate.
Last week, more than 21 people at the US detention facility in Cuba were on hunger strike.
The ICRC said on Wednesday that it had initially planned to send its team to Guantanamo on 1 April.
"However, in an effort to better understand current tensions and the ongoing hunger strike, we have decided to start this visit one week earlier," ICRC spokesman Simon Schorno said.
He added that "tensions at Guantanamo are certainly related, in our view, to the uncertainty that's prevailing on the future of the detainees".
"We see a clear link between that and their emotional state."
Pentagon spokesman Maj Jeff Pool told the BBC that 31 detainees were currently on hunger strike.
He said that 11 of them were being fed liquid food through tubes and three were treated at hospital.
Maj Pool provided no comment on the latest visit by the ICRC, saying only that the group's delegations had been visiting Guantanamo about six times a year starting from January 2002.
Hunger strikes have happened frequently at the US military prison, but this is reportedly one of the longest and more widespread protests.
It started several weeks ago and the number of detainees joining the protest was growing steadily.
Human rights groups and lawyers representing the prisoners say it reflects growing frustration at the US military's failure to decide their future.
Nearly 100 of the detainees have been reportedly cleared for release but remain at the facility because of Congressional restrictions and also concerns of possible mistreatment if they are sent back to their home countries.
However, Guantanamo officials deny claims that the strike began after copies of the Koran were mishandled during searches of prisoners' cells.
The military detention centre opened in 2002 to hold suspects captured in counter-terrorism operations after the 11 September 2001 attacks in the US.