Pacific Investment Management Co., home to the world's biggest bond fund, is cutting holdings of Italian and Spanish government bonds amid a sharp rally in the two markets this month.

The money-management firm has "lightened up" in the past couple of weeks in the European debt as part of a broad reduction in exposure to riskier assets in the fixed-income markets, said Andrew Balls, Pimco's head of European portfolio management, in an interview with The Wall Street Journal on Wednesday.

Mr. Balls declined to specify the dollar amount Pimco has sold of Italy and Spain bonds. Pimco manages more than $2 trillion in global assets.

Mr. Balls said the rally in prices of Italian and Spanish debt, which recently sent yields on 10-year debt to their lowest levels since 2010, was driven by liquidity from major central banks, which has overshadowed the euro zone's economic and fiscal problems for the moment. Yields on bonds fall when prices rise.

"This central-bank-inspired rally has made the markets more expensive," said Mr. Balls. "Yields could go lower still in Spain and Italy but we have sold into the rally because we remain worried over the fundamentals in the euro zone."

Mr. Balls highlighted three risks facing the euro zone. The European Central Bank's function of being a lender of last resort remains untested, he said, adding that politicians in the region have made little fresh progress toward a banking and fiscal union over the past months, making it difficult to see the potential for improvement in the flagging European economy.

Mr. Balls said after his recent selling, Pimco's positions in Spanish and Italian debt in some portfolios now are "neutral to a bit underweight," while other portfolios' exposure still remains "neutral to a bit overweight."

Big decisions on Pimco's overall portfolio will be decided by the company's annual meeting next month, said Mr. Balls.

He added that yields in Spain and Italy need to rise significantly before he would consider becoming a buyer again. Pimco still holds no sovereign bonds issued by Greece, Portugal and Ireland even though these bonds have also rallied.

The last time Pimco made a shift in Spanish and Italian debt was last summer when the ECB President Mario Draghi pledged to do whatever it takes to preserve the euro currency. Pimco was among many investors who turned bullish on Italy and Spain debt as the 10-year yield on Italian bonds tumbled from a record high above 7% after Mr. Draghi's assurance.

Wednesday, the 10-year Italy bond yielded 4.01% while the 10-year Spanish bond yielded 4.3%.

Mr. Balls argued that major central banks' money-pumping, by both the Federal Reserve and the Bank of Japan, has driven private investors out of safe-haven bonds such as Treasurys and German government debt whose yields are trading near historic lows, and into riskier assets.

The hunger for yield has pushed up bond prices in many parts of the global fixed-income markets, making those bonds expensive to buy and diminishing prospective returns, said Mr. Balls.

Rallies in riskier debt along with renewed concerns about global economic growth, have prompted Pimco to dial back exposures in several fixed-income markets this year, said Mr. Balls. Pimco has cut holdings of corporate bonds, including those in the U.S. industrial and financial sectors since the start of the year.

Write to Min Zeng at min.zeng@wsj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires