The C rating in Fitch's assessment is only one step above default, bringing it in line with the Moody's current equivalent score of Ca.
"Further ratcheting up of sanctions and proposals that could limit trade in energy increase probability of a policy response by Russia that includes at least selective non-payment of its sovereign debt obligations," the ratings agency said.
On March 16, Russia is due to pay $US107 million in coupons across two bonds, though it has a 30-day grace period to make the payments.
The country's financial markets have been thrown into a turmoil by Western sanctions after it invaded Ukraine, raising significant concerns over its ability and willingness to service debt.
The ratings firm pointed to a presidential decree that could potentially force a redenomination of foreign-currency sovereign debt payments into local currency for creditors in specified countries.