A former aide to the UN's top war crimes prosecutor says Russia shielded Bosnian Serb wartime leader Radovan Karadzic from arrest in 1997.
Radovan Karadzic is accused of genocide and other war crimes
The former aide, Florence Hartmann, makes the allegation in memoirs published in France.
Ms Hartmann also claims a French move to arrest Mr Karadzic failed because the then US President Bill Clinton wanted Russia to be informed first.
Russia would not let the West arrest Mr Karadzic, she alleges.
Mr Clinton was supported by Britain and Germany, according to Ms Hartmann, who was spokeswoman for the UN tribunal's chief prosecutor, Carla del Ponte, for six years up to last year.
Mr Karadzic remains a fugitive. He and his military chief, Ratko Mladic, are wanted on genocide charges by the UN tribunal in The Hague.
In her book, Peace and Punishment, Ms Hartmann says Russia temporarily spirited Mr Karadzic to Belarus in November 1997 as Nato troops were patrolling in Bosnia.
According to Ms Hartmann, former French President Jacques Chirac told Ms Del Ponte that then Russian President Boris Yeltsin had warned he would send a plane to get Mr Karadzic out of Bosnia if necessary. Mr Yeltsin would never permit the arrest of Mr Karadzic, the book claims.
Mr Chirac was angry over the Bosnian Serbs' capture of two French pilots in 1995, Ms Hartmann says, but Mr Clinton told him that an operation to arrest Mr Karadzic could not take place without Russia being informed.
Mr Clinton has not yet made any comments about Ms Hartmann's allegations.
Ms Hartmann also speculates that the West was worried about what Mr Karadzic might reveal about the 1995 Srebrenica massacre, in which Bosnian Serb forces murdered about 8,000 Bosnian Muslim men and boys.
The 1992-95 Bosnian War was ended by the Dayton peace accords, brokered by the US.
The US government has offered a reward of $5m for information leading to the arrest of Mr Karadzic or Gen Mladic.