The US and German governments have raised serious concerns over the way Russia has handled the investigation into alleged tax fraud at the country's largest oil company, Yukos. They have asked for assurances over the legality of the arrest a week ago of the Yukos chief, Mikhail Khodorkovsky. The US State Department spokesman, Richard Boucher, said Moscow needed to dispel concerns that the case against Khodorkovsky was politically motivated. Khodorkovsky, believed to be Russia's richest man, has been providing financial help to liberal opposition groups in Russia. Correspondents say it is widely assumed that is why President Vladimir Putin's government has taken legal action against him. A spokesman for German Chancellor Gerhard Schroeder said legal certainty was essential if Russia was to continue its integration into the world economy. Germany is Russia's largest trading partner and an important source of much needed foreign investment. Foreign advisers to Yukos had said on Friday that they would be lobbying Western governments on the firm's behalf. **PM 'concerned'*** On Thursday, Russian prosecutors froze almost half the shares in Yukos, though the freeze on some of those shares was lifted the following day. Russia's stock market has lost almost 15 per cent of its value over the past week. Analysts say Khodorkovsky's funding of opposition groups broke a tacit agreement to stay out of politics in return for avoiding investigation of his financial affairs. The head of the Russian parliament's budget committee, Alexander Zhukov, told the BBC on Saturday that, while he was bothered by the Yukos affair, it was unlikely to lead to any wider investigation of Russian businesses. "The president has already answered that... most likely this will be the only case," he told BBC Radio 4's Today programme. "It's not going to open up the door to everything." But on Friday, Prime Minister Mikhail Kasyanov broke ranks with the Kremlin by saying he was "deeply concerned" by events. His comments come amid speculation that the prime minister's position may ultimately be under threat. Kasyanov is a veteran of Russian politics, having risen to power under former President Boris Yeltsin in the climate that saw Yukos and other lucrative assets privatised in murky circumstances. Putin's chief of staff Alexander Voloshin, a fellow Yeltsin-era appointee, was replaced on Thursday amid reports that he was angry at the judicial campaign against Yukos. The BBC's Ian MacWilliam says Putin has sought to play down fears that the case heralds a wider campaign against the so-called oligarchs - the small group of super-rich businessmen who grabbed control of the country's most profitable industries during the privatisation of the 1990s. But some analysts say the events surrounding Yukos have created the biggest political and economic crisis of Putin's three years in office. Investors are reassessing Putin's credentials as manager of one of the world's biggest emerging economies, while political commentators are interpreting the action as a drive by Kremlin hardliners to suppress political dissent. "He's gone pretty far in tarnishing his reputation," said Andrew Kuchins, director of the Carnegie Moscow Centre think-tank. "It's not beyond the realm of possibility that he turns this around, but it will take a huge stroke of leadership to repair the damage." **Trading*** Russian prosecutors on Friday lifted the freeze on some of the blocked shares. The unfrozen shares - amounting to about 2 per cent of Yukos, or 4.5 of the 44 per cent that was earlier frozen - are said by prosecutors to belong to individuals unrelated to the fraud and tax evasion case against Khodorkovsky and key allies. Trading in Yukos shares remained volatile, while the wider stock market and the rouble steadied. Yukos shares closed 7.5 per cent higher on Friday, clawing back some of Thursday's losses. **PHOTO CAPTION*** Mikhail Khodorkovsky, seen at an October 20 press conference in Moscow. (Alexander Natruskin/Reuters)