China's investment, factory output and retail sales all grew more slowly than expected in April, adding to doubts about whether the world's second-largest economy is stabilising.

Growth in factory output cooled to 6 percent in April, the National Bureau of Statistics (NBS) said on Saturday, disappointing analysts who expected it to rise to 6.5 percent on an annual basis after an increase of 6.8 percent the prior month.

China's fixed-asset investment growth eased to 10.5 percent year-on-year in the January-April period, missing market expectations of 10.9 percent, and down from the first quarter's 10.7 percent.

Fixed investment by private firms continued to slow, indicating private businesses remain sceptical of economic prospects. Investment by private firms rose 5.2 percent year-on-year in January-April, down from 5.7 percent growth in the first quarter.

Reuters reported on Saturday that China's banking regulator has sent an urgent notice to banks telling them to clear bottlenecks holding back lending to private firms.

In its data announcement, the NBS said "Because the total amount of private investment is relatively large, the continued slowdown could restrain stable growth, and requires a high degree of attention."

Retail sales growth in April, which captures both private and government purchasing, slowed to 10.1 percent. Analysts forecast sales would rise 10.5 percent on an annual basis after gaining 10.5 percent the prior month.