China's relations with its ally Pyongyang have been further strained after a North Korean border guard shot four Chinese civilians, killing three of them.

Beijing has lodged a formal diplomatic complaint with North Korea over the shootings that also wounded a fourth Chinese civilian, Chinese Foreign Ministry spokesman Qin Gang said.

He gave few details to journalists at a regular briefing, saying only that the four were in North Korean territory last week and near the Chinese border town of Dandong. Neither side has said what the men were doing in North Korea.

But the men had been shot "on suspicion of crossing the border for trade activities," Qin said. "China attaches great importance to that and has immediately raised a solemn representation with the DPRK."

The nearly 900 miles of border are a known crossing for black market Chinese traders moving in and out of North Korea selling their wares, allegedly including illegal drugs.

Dandong, with a population of around 800,000, is across the river from Sin?iju city in North Korea. The two cities are connected by the China-Korea Friendship Bridge and there is a museum in Dandong dedicated to the "War to Resist U.S. Aggression and Aid Korea."

Chinese aid has for many years officially supported the economy of the struggling hard-line Communist state on its eastern border with food, fuel and manufactured goods.

What makes the shootings especially sensitive for Beijing is they come as North Korea faces more economic sanctions from the international community over its alleged sinking of the South Korean naval ship Cheonan.

An independent international investigation team concluded late last month that it was a North Korean torpedo that sank the 1,200-ton ship in the Yellow Sea on March 26, killing 46 sailors.

North Korea has consistently denied it had anything to do with the sinking and China has stood by North Korea, calling for international restraint.

But China also suggested to the North Korean leadership in Pyongyang that it do more than just reel off propaganda condemning South Korea and its allies, especially the United States. More hard evidence is needed to show the innocence of North Korea.

The border with North Korea has been the scene of several diplomatic incidents in recent years.

U.S. citizens Laura Ling, 32, and Euna Lee, 36, reporters for former U.S. Vice President Al Gore's Current TV, were detained by North Korea along the border with China in March 2009. They were accused of illegally entering the country from China and engaging in "hostile acts."

Both were tried and sentenced in North Korea to 12 years hard labor in June last year but freed in August after a mission by former President Bill Clinton.

In December 2009, religious activist Robert Park, 28, was arrested by North Korea after walking across the frozen Tumen River. Park, a U.S. citizen of Korean ancestry from Tucson, claimed at the time he had had a vision from God that he could help highlight religious persecution in North Korea.

He was released in February after confessing his "mistake" to North Korean officials and said freedom of religion was guaranteed in North Korea.