By Josie Cox 

European stocks remained volatile Friday, with investors weighing the prospect of more monetary stimulus in Europe against the threat of political upheaval in Greece as well as the Russian currency crisis.

Having opened the day higher, mirroring a surge on Wall Street on Thursday, the Stoxx Europe 600 was trading around 0.3% lower in the early afternoon.

On Thursday, the index ended the session up almost 3%, spurred at least in part by the U.S. Federal Reserve's pledge to adopt a "patient" approach when raising interest rates, but traders said that investors may have overshot, considering there are still several underlying sources of volatility.

Barclays economists noted that market moves in Russia have over the last few sessions "evolved from a commodity-focused correction to a correlated selloff." Even though comparisons with the 1998 default crisis may be exaggerated for now, they added, the situation remains "difficult."

The ruble on Friday continued to recover from its selloff earlier in the week, but still remains volatile. Sberbank strategist Tom Levinson said that the selling "frenzy" may have receded for now, but the pressure is here to stay.

Earlier this week the Russian finance ministry said it was ready to sell as much as $7 billion to stabilize the ruble after the central bank carried out massive monetary tightening, hiking its key interest rate by 6.5 percentage points to 17%.

Ultimately though, strategists agree that the fate of the Russian economy depends on the price of oil.

On Friday, Brent crude was trading just over $60 a barrel, having recovered somewhat from the 5- 1/2 -year low of $58.50 hit earlier this week. Year-to-date, the commodity has still tumbled more than 45% and Credit Suisse economists predict that the floor may be as low as $55.93/54.26 per barrel

Other currencies of oil-dependent economies, like the Canadian dollar and the Norwegian krone, were broadly unchanged.

Nigeria's naira remained close to an all-time low against the U.S. dollar after the country's central bank imposed new foreign-exchange controls on Wednesday. The bank barred dealers from depositing their currency-trading funds overnight, preventing traders from placing bets for or against a single currency at the close of a session.

Back in Europe, the equity selloff was also being spurred by political uncertainty in Greece as well as ratings agency Standards and Poor's on Thursday trimming its rating on several Italian lenders including two of the country's largest banks, citing its recent downgrade of the sovereign's rating from BBB to BBB-.

Shares in UniCredit SpA and Intesa Sanpaolo SpA were both down more than 3% after S&P cut both to BBB- from BBB. Milan's main stock index was 1.3% lower in the early afternoon, lagging all other major indexes in the region.

S&P said that the move reflects its view that economic prospects in Italy are likely to remain weaker over the next couple of years than they had previously anticipated.

Despite Friday's lull though, European stocks have proved relatively resilient this week, which economists and investors attribute to confidence that the European Central Bank would announce a broadening of its asset-purchase program to include sovereign bonds early next year.

In an interview with The Wall Street Journal earlier this week, ECB executive board member Benoît Coeuré sent one of the clearest signals to date that the bank is poised to embark on large-scale asset purchases early next year, as it grapples with a weak economy and dangerously low inflation.

"The ECB has fired all its monetary shots except [sovereign] quantitative easing. We are at the point now where QE is the most likely next step," said Andrew Bosomworth, head of portfolio management in Germany at Pacific Investment Management Co., which manages around $1.87 trillion in assets globally.

UBS chief European economist Reinhard Cluse said that so far his base-case scenario had been that the ECB will announce sovereign QE in March.

"But, should the December inflation data disappoint on the downside, this could, in our view, increase the likelihood that the ECB might announce sovereign QE as early as January," he added.

Earlier Friday, data showed that German consumer sentiment has hit its highest level in eight years, on hopes economic expansion in Europe's largest economy will accelerate in the coming months.

Write to Josie Cox at josie.cox@wsj.com

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