Growth in China's manufacturing sector picked up more than expected in March as authorities lifted winter pollution restrictions and steel mills cranked up production as construction activity swings back into high gear.

The official Purchasing Managers' Index (PMI) released on Saturday rose to 51.5 in March, from 50.3 in February, and was well above the 50-point mark that separates growth from contraction on a monthly basis.

Analysts surveyed by Reuters had forecast the reading would pick up only slightly to 50.5.

The findings add to a growing amount of data which suggest that China's economy has carried more momentum into the first quarter from last year than analysts had expected, which should keep synchronized global growth on track for a while longer even as trade tensions build.

February's print had been the lowest in 1-1/2 years, but many analysts suspected it was due to disruptions related to the long Lunar New Year holidays, not a sharp drop in consumption.

Indeed, the March survey showed manufacturers shifted into higher gear as usual as seasonal demand picked up at home and abroad. The sub-index for output jumped to 53.1 from 50.3 in February, while total new orders rose to 53.3 from 51.0 and export orders climbed to 51.3 from 49.0.

The China Logistics Information Centre, in a commentary on the PMI figures, said it expected first-quarter economic growth to be about 6.8 percent. Early this year, economists polled by Reuters were pencilling in a fade to around 6.6 percent.

Large companies saw a modest pickup in growth, while small firms' activity expanded marginally after shrinking in February.

Helping drive positive sentiment, exports have been better than expected in the first two months of the year, particularly for tech products, the fastest-growing segment of China's industrial sector. Though a sub PMI for hi-tech manufacturing eased in March, growth remained solid.

However, a sharp escalation in trade tensions with the United States is clouding the outlook for both China's "old economy" heavy industries and "new economy" tech firms.

The Trump administration slapped hefty tariffs on steel and aluminium imports last week and then targeted China specifically with plans for additional tariffs of up to $60 billion of its goods, likely focusing on tech and telecommunications products.

"Stress tests have shown the new U.S. tariffs will have a relatively small impact on Chinese steel. Chinese steel firms should not be overly worried and should focus on guaranteeing demand from the domestic market and our major exporters," the China Steel Logistics Professional Committee said.

"But it's worth noting that the amount of steel products we supply to U.S. consumers through the global supply chain may well exceed China's direct exports to the United States," it added. "China should proactively oppose U.S. unilateral trade protectionism to maintain the global supply chain."