Israeli Prime Minister Ariel Sharon is in a "serious" condition after a major stroke and faces hours more delicate neurosurgery, doctors have said.
Mr Sharon was due to have a minor heart operation on Thursday
Following an overnight operation to stem severe bleeding on the brain, he was taken back into surgery as doctors examined results of a scan.
The 77-year-old was taken to hospital in Jerusalem on Wednesday evening.
He was treated for a minor stroke in December. Deputy Prime Minister Ehud Olmert has assumed Mr Sharon's powers.
Israel waited through the night for news of Mr Sharon's health, before the director of Jerusalem's Hadassah Hospital delivered a short statement just after dawn.
Shlomo Mor-Yosef confirmed that Mr Sharon had suffered a "significant" stroke and subsequently required surgery to stop bleeding on his brain - effectively a brain haemorrhage.
Mr Sharon, who was prescribed blood thinning drugs after December's stroke, required new medication in addition to surgery to regulate his blood clotting.
After lengthy overnight surgery doctors carried out a brain, or CT, scan, to assess the effects of the operation, Mr Mor-Yosef said.
"The prime minister has [now] been taken back to the operating theatre and we are now continuing the same operation as there are additional areas that have to be treated," he added.
"The surgery is going to be continued for the next few hours. Part of it will be surgical, part of it will be to give the different medications a chance to act."
Mr Sharon remains under anaesthetic and is connected to a respirator, the director said.
"The situation is serious."
Overnight, Mr Sharon's press spokesman Raanan Gissin described his condition as "much more serious" than his previous stroke.
That condition could have been caused by a small hole in his heart, doctors suggested, which Mr Sharon was voluntarily due to have treated on Thursday.
Instead the prime minister was carried to hospital in Jerusalem from his ranch in the Negev Desert in Israel's south by ambulance. He was reported to be awake during the journey and did not travel by helicopter.
Police and security agents set up a security cordon around the hospital, and also stationed themselves around Mr Olmert's residence in Jerusalem.
The deputy leader is due to chair a cabinet meeting on Thursday to brief ministers about Mr Sharon's condition.
US President George Bush said in a statement he shared the concerns of the Israeli people "and we are praying for his recovery".
The BBC's James Reynolds, in Jerusalem, says however the situation develops now, even if Mr Sharon makes a recovery, there will be serious questions over his health and whether he can continue to lead the country.
Mr Sharon, who has been prime minister since 2001, is severely overweight.
He had been planning to run for a third term in office under his newly formed centrist party, Kadima, after quitting the ruling Likud party in November.
Polls had suggested his new party is in the lead ahead of the election in March.
However, David Horowitz, editor of the Jerusalem Post, said Mr Olmert does not have the charisma of Mr Sharon or the ability to carry his controversial policies through.
Mr Sharon was thought likely to follow the unilateral withdrawal of settlers from the Gaza Strip with significant "painful concessions" in the West Bank.
The Kadima party "is united solely around Sharon... it will be much less electable without Sharon," Mr Horowitz said.