Some 84 prisoners are now refusing food, with 46 of them reported to have joined the protest on 25 December. The US military defines a hunger strike as missing nine consecutive meals. Human rights groups have challenged the US in the past over the number of detainees it says are refusing food and whether they have been force fed.
About 500 prisoners remain at Guantanamo, many of them captured in Afghanistan. Some have been held for nearly four years without charge.
Guantanamo spokesman Lt Col Jeremy Martin said: "The number of detainees involved in the current fast, which began on 8 August 2005, routinely fluctuates. "On the anniversary of 11 September, the number of strikers spiked to 131. "They steadily decreased over the weeks and months until 25 December, and then they spiked again."
The BBC's Adam Brookes in Washington says the military will not give any details concerning the strikers' medical condition. In a statement, the army said it was providing appropriate nutrition through nasal tubes, a procedure that would be consistent with force feeding, our correspondent says.
Lawyers for some of the detainees have said the hunger strikers are protesting against their continued detention without trial and against the conditions in which they are being held, he adds. Human rights campaigners have expressed growing concern about the treatment of inmates at Guantanamo.
UN officials - who have been trying to visit the camp since it opened in January 2002 - formally rejected a US invitation in November because too many restrictions were imposed. The Bush administration has denied allegations of abuse at Guantanamo, insisting it does not torture prisoners.