Rosneft executive Sergei Bogdanchikov said in Kazakhstan Wednesday that the assets frozen include a 20 percent stake in Sibneft, a firm Yukos had planned to merge with in 2003.

The Itar-Tass news agency quotes Mr. Bogdanchikov as saying Rosneft is trying to reclaim $3.5 billion.

Rosneft assumed debt from a key Yukos production unit it acquired after the unit was sold in a disputed government auction last year. 

Yukos has been the target of an intense wide-ranging government probe.

Critics accuse the Kremlin of persecuting the firm for former Yukos chief Mikhail Khodorkovsky's support of the political opposition.

The Kremlin says Khodorkovsky committed massive fraud.  The former Yukos head was recently sentenced to nine years in a prison camp for tax evasion and fraud.