A powerful earthquake struck communities along a strategic mountain pass near the Great Wall in northern China on Saturday.

Officials with the State Seismology Bureau in Beijing said they were trying to determine the extent of casualties and damage from the magnitude-6 quake, which struck at 11:50 a.m. local time.Seismologists were having trouble contacting Shangyi County, 150 miles northwest of Beijing, and the city of Zhangjiakou. A team of experts was sent to assess damage, said one seismologist who refused to give his name.

The quake was centered in Shangyi County just north of the Great Wall in Hebei province, the Xinhua News Agency reported.

Local officials could not be reached by telephone for comment.

A magnitude 6 quake is capable of causing severe damage.

Zhangjiakou lies on a strategic pass in the Yan Mountains and for centuries was an important trading town linking Beijing, the imperial capital, with Mongolia. After the communists came to power in 1949, the town housed a large military garrison and only in recent years has it been opened to foreign travelers.