Uzbekistan is preparing to hold a funeral for president Islam Karimov, according to foreign diplomats.
Uzbek officials have said only that Karimov is gravely ill, but messages of condolence have started arriving from other governments.
An Afghan official said president Ashraf Ghani will attend Karimov's funeral on Saturday, and a senior Kyrgyzstan diplomat said the country's prime minister had been invited.
Iran's president Hassan Rouhani sent a message to Nigmatilla Yuldoshev, said to be the acting president of Uzbekistan, offering condolences for Karimov's death, according to a posting on Mr Rouhani's website.
Earlier, Turkish prime minister Binali Yildirim offered his condolences to the Uzbek people for Karimov's death, although it was not clear how he got the news.
Russian presidential spokesman Dmitry Peskov said the Kremlin had no confirmation that Karimov might be dead.
Karimov, 78, has not been seen in public since mid-August, and his government admitted last weekend that he was ill. His daughter Lola said he had suffered a brain haemorrhage, and a swarm of unofficial reports have placed him close to death or even dead.
Karimov has run an authoritarian government in the Central Asian nation since 1989, and cultivated no apparent successor.
After several days of silence, the government on Friday issued a statement saying: "Dear compatriots, it is with a heavy heart that we inform you that the health of our president has sharply deteriorated in the past 24 hours to reach a critical state, according to the doctors."
Uncertainty over his health has raised concerns that Uzbekistan could face prolonged infighting among clans over leadership claims, something its Islamic radical movement could exploit. The Islamic Movement of Uzbekistan has over the years been affiliated with the Taliban, al Qaida and Islamic State and has sent fighters abroad.
Under the constitution, if the president dies his duties pass temporarily to the head of the senate until an election can be held within three months. However, the head of the Uzbek senate is regarded as unlikely to seek permanent power and Karimov's demise is expected to set off a period of jockeying for political influence.
Denis Krivosheev, deputy director for Europe and Central Asia at Amnesty International, said Karimov's death would "mark the end of an era in Uzbekistan, but almost certainly not the pattern of grave human rights abuses. His successor is likely to come from Karimov's closest circle, where dissenting minds have never been tolerated".
Uzbekistan celebrated its Independence Day on Thursday, and it was widely assumed that the government would not break any news until after the festivities.
On Friday, indications mounted that the country was preparing for a funeral. Photographs posted by the respected Central Asian news website Fergana.ru showed what appeared to be undertakers in Karimov's home town of Samarkand working on a cemetery plot in the graveyard where his family is buried.
Samarkand airport announced it would be closed to all flights except specially approved aircraft on Saturday, according to the website of the US Federal Aviation Administration.