(Reuters) -Fitch on Wednesday downgraded Russia’s sovereign credit rating by six notches to “junk” status, saying Western sanctions over the invasion of Ukraine made it uncertain Russia could service its debt and would weaken its economy in “a huge shock” to its creditworthiness.

Russia’s financial markets have been thrown into turmoil by its assault on Ukraine, the biggest on a European state since World War Two, and stiff Western sanctions.

The invasion has triggered a flurry of credit rating moves and dire warnings about the impact on Russia’s economy. Last week, S&P lowered Russia’s rating to junk status and Moody’s put the country on review for a downgrade to junk.

The Institute of International Finance predicts a double digit contraction in economic growth this year.

Fitch downgraded Russia to “B” from “BBB” and placed the country’s ratings on “rating watch negative”.

“The severity of international sanctions in response to Russia’s military invasion of Ukraine has heightened macro-financial stability risks, represents a huge shock to Russia’s credit fundamentals and could undermine its willingness to service government debt,” Fitch said in a report https://www.fitchratings.com/research/sovereigns/fitch-downgrades-russia-to-b-on-rating-watch-negative-02-03-2022.

Fitch said that U.S. and EU sanctions prohibiting any transactions with the Central Bank of Russia would have a “much larger impact on Russia’s credit fundamentals than any previous sanctions”, rendering much of Russia’s international reserves unusable for FX intervention.

“The sanctions could also weigh on Russia’s willingness to repay debt,” Fitch warned. “President Putin’s response to put nuclear forces on high alert appears to diminish the prospect of him changing course on Ukraine to the degree required to reverse rapidly tightening sanctions.”

Fitch said it expects further ratcheting up of sanctions on Russian banks.

The sanctions imposed by Western countries will also markedly weaken Russia’s GDP growth potential relative to the ratings agency’s previous assessment of 1.6%, Fitch said.

Sanctions imposed on Russia have significantly increased the chance of the country defaulting on its dollar and other international market government debt, analysts at JPMorgan and elsewhere said on Wednesday.

Russia has responded to the sanctions with a range of measures to shore up its economic defenses and retaliate against Western restrictions. It hiked its main lending rate to 20%, banned Russian brokers from selling securities held by foreigners, ordered exporting companies to buttress the rouble and said it would stop foreign investors selling assets.

The government also plans to tap its National Wealth Fund (NWF), a rainy day cushion, to help counter sanctions.

(Reporting by Mehr Bedi in Bengaluru and Megan Davies in New York; Editing by Maju Samuel and Grant McCool)