Mr Aleksanyan (right) says the accusations against him are absurd

Vasili Aleksanyan, the oil firm's most senior executive still resident in Russia, was seized by Russian police on Thursday and is still being held.

Mr Aleksanyan, appointed to protect Yukos assets as it tries to stave off bankruptcy, is suspected of money laundering but has yet to be charged.

Yukos has fought a three year legal battle with the Russian authorities.

Crippling payments

Its finances have been crippled by demands for $32bn (�18.4bn) in back tax payments.

Russian prosecutors say the firm deliberately avoided paying taxes but Yukos claims their campaign has been politically motivated.

Former Yukos chief executive Mikhail Khordokovsky is currently serving eight years in a Siberian prison after being convicted of fraud and tax evasion.

We can only assume that this action is a direct result of his accepting a position to work to protect Yukos Yukos Timeline: The rise and fall of Yukos

The firm faces potential bankruptcy after a group of banks took it to court to recover $480m in loans which they say Yukos defaulted on.

Mr Aleksanyan, a former Yukos lawyer, was recently rehired to oversee the firm's remaining assets during bankruptcy hearings which began in early March.

Police seized Mr Aleksanyan from his apartment , but although prosecutors confirmed his arrest they gave no further details.

'Inappropriate action'

In a statement, Yukos said they hoped for Mr Aleksanyan's early release and criticised the way in which he was arrested.

"We can only assume that this action against him is a direct result of his accepting a position to work to protect Yukos and its legitimate stakeholders," the firm said.

Chief executive Steven Theede - who along with most Yukos bosses is based in London - said he was concerned about Mr Aleksanyan's health.

"Those who wish harm for Yukos have learned that actions that would normally be considered inappropriate when effected against Yukos will be met with essentially no repercussions from inside or outside Russia," he said.

Yukos lost control of its main oil producing unit in 2004, which was ultimately acquired by the energy giant Rosneft.