Former Guantanamo detainee Abu Wael Dhiab is weak from a hunger strike and could need to be treated in hospital, according to a friend.
Dhiab, who suffers health problems related to his hunger strikes and forced feedings while in US custody, has repeatedly said that he is unhappy in Uruguay, which took him and five other former Guantanamo prisoners in for resettlement in 2014.
He has been demanding that he be allowed to leave the South American country.
It is not clear when Dhiab began his hunger strike, but a friend, Jorge Voituret, said the former prisoner stopped drinking water Friday.
"He's very weak and it's possible that in the next hours he'll have to be treated at a hospital," said Voituret, who works for Uruguay's Memory Museum, an organisation that documents human rights violations committed during the country's 1973-1985 military dictatorship.
Dhiab, a Syrian, went missing from Uruguay for several weeks earlier this year, alarming officials in neighbouring countries and setting off recriminations from US politicians before he resurfaced in Venezuela.
The Uruguayan foreign ministry has said that while he was in Caracas, Dhiab tried to get help to reunite with his family in Turkey or another country before he was deported back to Uruguay last week.
Dhiab's liaison with Uruguay's government announced on Thursday that Dhiab planned to go on a hunger strike in the hope of being allowed to go elsewhere.
Since his return to Montevideo, Dhiab has grown frustrated with a Uruguayan government-appointed NGO that owes him three months of aid, Voituret said.
"He planned to use the money to buy a telephone and a stove. He is living in an apartment with no heating, and it gets really cold these days," the friend said about the southern hemisphere's winter months.
Gabriela Cortina, an official at the Ecumenical Services for the Human Dignity, said the NGO would not comment.
Former President Jose Mujica invited Dhiab and the other former detainees - three more Syrians, a Tunisian and a Palestinian - to resettle in Uruguay.
They had been arrested in 2002 for suspected ties to al-Qaeda and held without charge like hundreds of others at the US military's Guantanamo Bay base before American officials cleared them for release.
Uruguay's government has provided social services and financial support. But the men have struggled to adjust and have protested that they don't get enough help.
Dhiab has been the most vocal about his unhappiness. Shortly after arriving in Uruguay, he called a news conference to complain that the government needed a better resettlement plan.