The 93 former president had been sedated and was breathing with the aid of a respirator, his office said.
Israel's former President and Nobel Peace Prize winner Shimon Peres has been put into an induced coma in a hospital after he suffered a stroke on Tuesday.
His office initially reported that "his condition is stable and he is fully conscious". But, later it said that Peres, 93, had been sedated and was breathing with the aid of a respirator.
"Former president Peres' doctors sedated and intubated him so as to best facilitate the continuation of his treatment," his office said.
"He will undergo a CT scan so as to get a full and updated assessment of his situation."
Get Well Shimon Peres, Man of Peace— Christiane Amanpour (@camanpour) September 13, 2016
Speculation mounted over his condition late Tuesday in Israel, with local media reporting that he was in serious condition.
"Shimon, we love you and the entire nation is wishing for your recovery," Prime Minister Benjamin Netanyahu said in a statement.
"The prime minister conveyed the prayers of the entire nation for a quick recovery," his office said.
In January, Peres was hospitalised twice for heart trouble.
In the first instance, the hospital said he had suffered a "mild cardiac event" and underwent catheterisation to widen an artery.
He was rushed to hospital a second time days later with chest pains and an irregular heartbeat.
A co-architect of the 1993 Oslo peace accords, Peres won the 1994 Nobel Peace Prize jointly with Israeli premier Yitzhak Rabin, who was assassinated the following year, and then-Palestinian president Yasser Arafat.
Peres has sought to maintain an active schedule despite his age, particularly through events related to his Peres Center for Peace.
When leaving hospital on January 19, Peres said he was keen to get back to work.
"I'm so happy to return to work, that was the whole purpose of this operation," he told reporters.
In a career spanning nearly seven decades, Peres, 93, served in a dozen cabinets and twice as a Labour Party prime minister, even though he never won a general election outright in five tries from 1977 to 1996.
He later served as president, a largely ceremonial role in Israel, from 2007-2014, before leaving government.
Secret to longevity
Peres had once confided that the secret to his longevity was daily gymnastics, eating little and drinking one or two glasses of good wine.
He once hawkishly rejected any compromise with Arab states, but said he was converted after 1977, when Egyptian president Anwar Sadat made a historic visit to Jerusalem, leading to the first Arab-Israeli peace treaty.