BEIJING - China's railway freight volume, an indicator of economic activity, slumped further in the first half of 2015, the country's top economic planner said on Friday.

The railways transported 1.7 billion tons of cargo in the first half, down 10.1 percent year on year, compared with a decrease of 9.4 percent in the first quarter, according to data released by the National Development and Reform Commission.

The sluggish figure came as another key index for manufacturing activity reported the biggest drop for 15 months in July, showing the foundation for China's economic recovery is not stable yet.

The country's Purchasing Managers' Index (PMI) retreated from 49.4 in June to 48.2 in July, according to results of a private survey published on Friday. A reading above 50 indicates expansion, while a reading below that represents contraction.

China's economy posted a 7-percent growth year on year in the second quarter, unchanged from the first quarter, but better than market predictions.