MOSCOW (CNN) -- President Boris Yeltsin left the hospital where he had been recovering from pneumonia Monday, fired key aides at the Kremlin, then returned to the hospital only hours later.

The shakeup removed several top aides who had publicly questioned the president's health in recent weeks.

Yeltsin immediately returned to the government hospital where he has been treated for the past two weeks, to continue his recovery.

Yeltsin dismissed Chief of Staff Valentin Yumashev and his deputies Yuri Yarov, Mikhail Komissar and Yevgeny Savostyanov, a Kremlin spokesman said.

Nikolai Bordyuzha, the secretary of the presidential security council, was appointed the new chief of staff, while the other posts remain vacant. Bordyuzha, who will retain his security job, had been chief of the Border Guards before his appointment to the post earlier this year.

Asked when Yeltsin would be released, presidential spokesman Dmitry Yakushkin said it would depend on the treatment and how long it took the president to recover.

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Yeltsin has a habit of unexpected and often unexplained dismissals. The latest round of dismissals was thought to reflect his unhappiness over the aides' unusual candidness about his health in recent weeks.

Yumashev and other Kremlin aides openly said that Yeltsin is too weak to handle day-to-day affairs or to make foreign trips and suggested he leave control of the economy and other current affairs to Prime Minister Yevgeny Primakov.

Yeltsin, who has a history of health problems, has been little involved in the day-to-day management of Russia since late summer, and has spent most of his time at country residences. On November 22 he was admitted to the hospital with pneumonia.

Yakushkin said Yeltsin's blitz appearance in the Kremlin was prompted by fears his administration was showing a lack of commitment in fighting corruption, political extremism and regional separatism amid a bitter economic crisis.

In the shakeup, Yeltsin removed three of Yumashev's deputies and the head of the secretive government communications agency FAPSI.

Political opponents have accused Yeltsin of being unable to control Russia and have urged him to step down early. But the Kremlin has said Yeltsin intends to serve his full term until mid-2000.

The Associated Press and Reuters contributed to this report.