By Sara Sjolin, MarketWatch

LONDON (MarketWatch) -- U.K. stocks trimmed losses in the morning on Friday after construction data for January beat expectations, partly offsetting wider worries about tensions in Ukraine and an economic slowdown in China.

The FTSE 100 index fell 0.1% to 6,544.41, on track for a sixth straight day of losses, which would be the longest losing streak since November 2011, according to FactSet data.

The U.K. benchmark, however, posted one of the smallest losses in Europe, supported by data showing construction output in January rose 1.8% month-on-month. HSBC analysts expected a rise of 1.2%. The Office for National Statistics said the longer-term data also showed an improved picture for the sector, with both new work and repair and maintenance rising on the year.

The better-than-expected data weren't, however, enough to lift the index out of the red, as the broader investment sentiment in Europe was dented by nervousness ahead of Sunday's referendum in Ukraine that's likely to see citizens of the Crimea region vote to return the Black Sea peninsula to Russia. The Ukrainian government, the U.S. and the European Union contend the referendum is illegal, and a vote in favor of rejoining Russia would likely be followed by a round of U.S.- and European-led sanctions against Moscow.

Meanwhile, ongoing concerns about slower economic growth in China have hit financial markets this week after a string of disappointing data, including weak exports and weaker-than-expected retail sales.

Mining firms, which are sensitive to Chinese growth and the wider risk appetite among investors, posted some of the biggest losses in London on Friday. Shares of Glencore Xstrata PLC (GLCNF) fell 1.3%, Rio Tinto PLC (RIO) lost 0.9% and Vedanta Resources PLC gave up 0.9%.

Wm. Morrison Supermarkets PLC dropped 0.5%, continuing its decline from Thursday, when the shares slid 12% after the food retailer issued its second profit warning in three months.

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