But ICRC chief spokeswoman Antonella Notari declined to comment on Thursday's statement by a defence lawyer that the hunger strike involved 200 of 500 prisoners and that 21 were being force-fed.
The humanitarian agency, which last visited the US naval base in Cuba in late September, was in contact with the US authorities about the situation, Notari said on Friday.
"There is a hunger strike, the situation is serious, and we are following it with concern," Notari said.
"During our recent 10-day visit we were able to visit the infirmary, see the detainees and speak with them as well as the American authorities," she added.
The ICRC backs a 1975 Tokyo declaration by the World Medical Association stating that doctors should not participate in force-feeding, but keep prisoners informed of the sometimes irreversible consequences of their hunger strike, she added.
"There is a hunger strike, the situation is serious, and we are following it with concern"
Chief ICRC spokeswoman
Amnesty International and human-rights lawyer Clive Stafford Smith, representing 40 detainees, said on Thursday that US authorities were keeping 21 alive by forcing food into their stomachs through tubes pushed up their noses.
The prisoners are shackled to their beds 24-hours a day to stop them removing the tubes, he said.
"This is the 56th day of the hunger strike," said Stafford Smith before making a comparison with the Irish republican campaign of 1981, when 10 prisoners starved themselves to death in protest at British policy in Northern Ireland.
The US opened the prison camp in January 2002. Many detainees were seized in Afghanistan. Only four have been charged and many have been held for more than three years. Some former prisoners says they were tortured while in Guantanamo.
Force-feeding is not banned under international law, but the World Medical Association declaration, endorsed by the American Medical Association, sets guidelines for doctors involved in hunger strikes and says they should not participate in force-feeding.