Russian prosecutors have lifted a freeze on some shares in Russia's largest oil company, Yukos, amid growing international and domestic concern.

Russian prosecutors say they decided to release nearly five percent of the controlling stake of shares they seized Thursday, because they found that the shares belong to individuals not implicated in the criminal investigation against senior Yukos executives.

The announcement follows a statement by Russian Prime Minister Mikhail Kasyanov, who said he is 'deeply concerned' about the seizure of the shares.

Mr. Kasyanov told journalists the seizure of shares in a private company traded on the Russian stock market is a new phenomenon, the consequences of which are difficult to define.

Mr. Kasyanov made the comment to reporters in the southern city of Nalchik, just a few hours after Russian prosecutors seized more than 50 percent of Yukos shares. President Vladimir Putin had urged members of his Cabinet not to be drawn into the Yukos affair.

Friday's developments come the day after President Putin accepted the resignation of his chief of staff, Alexander Voloshin, who was seen as sympathetic to arrested Yukos Chief Executive Mikhail Khodorkovsky, who is jailed on seven charges of tax evasion and fraud.

Mr. Voloshin's departure was widely expected. He had been reported to be increasingly wary of the government's handling of the dispute with Yukos and its senior executives. Mr. Voloshin and Prime Minister Kasyanov were among the last senior officials remaining from former President Boris Yeltsin's administration.

Still, the news was met with concern by leading opposition politicians like parliament member Irina Khakamada.

Ms. Khakamada told Russian television that Mr. Voloshin's resignation is very significant. She says it tells her that the balance of power between two rival groups inside the Kremlin has been lost, and that trouble lies ahead.

Confirmation of Mr. Voloshin's resignation came late Thursday, just a few hours after news broke that the Russian prosecutor's office had seized a majority of the shares in Yukos. As a result, prices on Russia's financial markets tumbled for the second time in a week.

Russian markets recovered somewhat on Friday, including shares in Yukos. But the company's stock price is still far below its past level, because of a drop of 20 percent after Mr. Khodorkovsky's arrest.

Market analysts say investors are apparently taking some comfort in President Putin's announcement that pro-business moderate Dmitri Medvedev will replace Mr. Voloshin as chief of staff.

Like Mr. Putin and many of his Kremlin aides, Mr. Medvedev comes from St. Petersburg. But unlike many of them, he is not known to have a KGB past. The new chief of staff is a lawyer and businessman of note, having served as chairman of Russia's huge national gas company, Gazprom. Previously, he served as first deputy chief of Mr. Putin's staff.