* Bank has been studying withdrawing from Russia

* CEO says conclusion will take some time

* Chair says bank faces most challenging year ever

VIENNA, March 31 (Reuters) - Austria's Raiffeisen Bank International, one of the banks in Europe most exposed to Russia, said on Thursday the group was not at risk of contagion even in an extreme scenario at its Russian subsidiary.

The statement is aimed at allaying fears that the entire lender could be sucked into a downward spiral as a result of the crisis sparked by Russia's invasion of Ukraine.

RBI said in a presentation for its annual general meeting that it was built to "ensure no spill-over to the rest of the group even under any extreme scenario".

"The exposure to Russia is well within the limits which ensure resilience of the group under all possible scenarios," it said.

RBI has been studying a possible withdrawal from Russia.

The bank's chief executive Johann Strobl said reaching a conclusion would take some time. "A bank isn't a sausage stand that can be closed in a week," Strobl told shareholders.

The lender has operated in Russia since the collapse of the Soviet Union 30 years ago, and its business there, Russia's No. 10 bank by assets, contributed almost a third to the group's net profit of 1.5 billion euros ($1.67 billion) last year.

RBI is facing its most difficult year since its foundation, its chair Erwin Hameseder told shareholders.

The bank's shares have fallen 38% since the invasion, which Russia has called a "special operation". The shares were trading 1.3% lower on Thursday morning.

In a nod to the hardship faced by employees in Ukraine, Strobl said some were working in metro stations and garages, but the bank has kept open as many branches in the country as possible.

($1 = 0.8958 euros) (Reporting by Alexandra Schwarz-Goerlich; Writing by Tom Sims; Editing by Miranda Murray, Madeline Chambers and Jan Harvey)