The price of iron ore has slipped for a second session, trending back toward $US50 a tonne after recently enjoying a surge beyond $US55 for the first time in a month.

At the end of the latest session, benchmark iron ore for immediate delivery to the port of Tianjin in China was trading at $US52.90 a tonne, down 3.1 per cent from $US54.60 a tonne in the prior session.

The fall has come as the Shanghai stock exchange again drifts lower, losing 1.1 per cent on Friday to put an exclamation point on a 15 per cent drop for the month.

Weakness on the key Chinese exchange has many wondering if it is a signal of more worrying issues with the world's second largest economy.

Mixed data over the weekend will do little to ease concerns, with the manufacturing PMI slipping to 50 in July, slightly below market expectations.

In the short-term traders are also worried about demand given a temporary Beijing crackdown on air pollution in September.

The move will likely involve limits on steel production in Hebei province, which surrounds major iron ore port Tianjin.

“Steel mills near Beijing ... will probably be forced to shut down production again,” Macquarie said in a research note last week. “This clearly spells trouble for iron ore prices.”

The action is designed to improve air quality during World War II commemorations.

The iron ore price reached a one-month higher in the middle of last week on reports of a slowdown in Australian exports as well as news of rising steel prices.