Ze'ev Rothstein, director of Sheba Medical Center at Tel Hashomer near Tel Aviv, said Sharon, reportedly suffering from serious kidney failure, was not on dialysis but receiving antibiotics because of numerous infections, the Jerusalem Post reported.
"He's getting all the treatment necessary," Rothstein said.
Sharon, 85, has experienced "a serious deterioration in his health" during the past two days, Rothstein said.
Sheba Medical Center spokesman Amir Marom said Sharon's condition has worsened during the past several days.
Sharon's sons, Omri and Gilad, were with their father. Doctors at Sheba reportedly told the sons their father likely has a few days left to live, the Post said.
"We have hope, we always have hope," Omri Sharon, a former lawmaker, told the Post from the hospital.
U.S. Secretary of State John Kerry, in Jerusalem to meet with Prime Minister Binyamin Netanyahu, said the United States' thoughts and prayers were with the Sharon family as they sit vigil with the former prime minister.
"We remember his contributions, sacrifices he made to ensure the survival and the well-being of Israel, and I have many personal thoughts about my meetings with him on many different occasions – always robust and strong and clear about his positions," Kerry said. "[All] Americans are thinking of Israel and their ... former leader, and of the vigil that is taking place now at a very personal level with his family."
Sharon had been undergoing dialysis for kidney failure to prevent the rest of his organs from failing, the Post said. He underwent surgery a month and a half ago to treat an infection caused by kidney failure.
Sharon has been in a coma since his Jan. 4, 2006, stroke.
Justice Minister Tzipi Livni, who founded the Kadima party with Sharon, posted on Facebook: "I can't stop thinking about Arik [Sharon] tonight."
Sharon, one of Israel's legendary politicians and military leaders, played a key role in military victories in both the 1967 Six Day War and in the 1973 Yom Kippur War.
In the political realm, he was the father of the Likud and Kadima parties. Sharon held several Cabinet posts, including Defense, Construction and Housing, and the Foreign ministries. He was elected prime minister in 2001.