A Russian-born American has been named to replace Mikhail Khodorkovsky as chief executive of Russia's largest oil company.

Yukos said its board of directors had appointed Simon Kukes as chief executive, replacing Mr Khodorkovsky who resigned on Monday.

Mr Kukes, 56, emigrated from the Soviet Union to the US in the 1970s but has since returned to Russia.

Mr Khodorkovsky, Russia's richest man, is being held on charges of fraud and tax evasion.

Many Russians believe the case against Mr Khodorkovsky, who made his fortune through controversial privatisations in the 1990s, is politically motivated.

He has funded opposition groups, breaking what analysts say was a tacit agreement to stay out of politics in return for avoiding investigation of his financial affairs.

The crackdown on Yukos and its boss stirred fears of a wider confrontation between the Kremlin and big business.

Defence

The appointment of Mr Kukes follows a stubborn defence by Russian President Vladimir Putin of his government's record over the Yukos crisis.

He said the detention of Mr Khodorkovsky was neither evidence of authoritarianism nor a sign that Moscow was considering renationalising privatised industries.

Speaking to Italian journalists ahead of a state visit to Italy, Mr Putin said the Yukos case was about law enforcement.

"There will be no de-privatisation or revision of privatisation results," he said. "But one and all should learn the skill of living by the law and observing the law of the country."

He compared Mr Khodorkovsky's fate to that of the "score of US company bosses" arrested in the past two years.

"But nothing extraordinary happened," he told Italian daily Corriere della Sera, "and no one questioned the existence of the rule of law. For us too, it is nothing exceptional."

'Political ambition'

Shares in Yukos rose by 12% following the resignation of Mr Khodorkovsky.

The tycoon said he was stepping down from the oil company to protect his workforce.

He said he now planned to dedicate himself to his Open Russia Foundation - an organisation that he said was aimed at building an open and democratic Russia.

A BBC Moscow correspondent says the statement indicates Mr Khodorkovsky is preparing to bring forward his declared intention to enter the political arena and challenge those who he believes brought political charges against him.

'Good step'

By resigning from the company, Mr Khodorkovsky is effectively trying to disassociate Yukos from the charges of fraud and tax evasion that he faces.

In a statement, he said he "must do my utmost to lead our working team out from under the attack which has been directed against me and my partners".

A Russian stock market analyst, Michael Sito, says traders had widely welcomed the move.

"Investors saw that Mr Khodorkovsky has put a distance between himself and the company which is seen as a very good step," he said.