China formally protested on Tuesday that three of its citizens were killed and a fourth wounded by North Korean border guards who opened fire last week in an apparent attempt to crack down on smuggling.

The Chinese were from the border city of Dandong, site of the Friendship Bridge, across the Yalu River, commemorating China's support for the North during the Korean War. According to reports in the South Korean media, the Chinese were suspected of smuggling copper wire out of the North Korean city of Sinuiju, which is on the other side of the bridge. The reports said they were on a boat on the river when they were shot Friday.

"In the aftermath of the incident, China has paid a lot of attention to this issue and has made a formal diplomatic protest to North Korea," Chinese Foreign Ministry spokesman Qin Gang said, reading an official statement at a regular news briefing in Beijing.

The incident comes in the midst of a furor over the March 26 sinking of a South Korean naval ship that killed 46 sailors. At least publicly, China has refused to take sides, angering South Korean and U.S. officials who say there is overwhelming evidence that an unprovoked North Korean torpedo attack caused the ship to go down.

The irony of China's protest over last week's shooting was not lost on South Korea.

"This time it is their citizens who are killed, and they show they are not so naive after all about North Korea," said Kim Tae Jin, a North Korean defector and human rights activist in Seoul. However, he applauded China's protest of the shooting. China needs to show North Korean leader Kim Jong Il "that he can't get away with whatever he wants," Kim said.

China's public protest is unusual in that relations between China and North Korea are normally shrouded in secrecy, to be discussed only in the politburos of the longtime communist allies.

"It is rare for China to publicly complain. Usually there is a private apology or money paid," said Kim Heung Gwang, a former North Korean college professor and head of Seoul-based North Korea Intellectuals Solidarity.

The stretch of the Yalu just south of Dandong is frequently trafficked by smugglers, some of them bringing North Korean-made drugs into China or banned Chinese products, such as DVDs or cellphones, into North Korea.

The North Korean government is especially strict about the export of copper, which has been looted from factories, electrical and telecommunications facilities by Northerners desperate for money. But the North's border guards do not normally shoot to kill — at least not when the smugglers are Chinese.

"Only their own people," said Kim.

Tensions remain high in the region over the sinking of the South Korean ship. The Global Times, an English-language newspaper with close ties to the Chinese Communist Party, on Tuesday complained about joint U.S.-South Korean naval exercises planned for the Yellow Sea, where the Cheonan went down. Some reports said the George Washington, a U.S. aircraft carrier, would participate, although the Pentagon said a decision had not been made.

"Though intended to send a threatening message to North Korea, having a U.S. aircraft carrier participating in joint military drills off of China's coast would certainly be a provocative action toward China," the newspaper editorialized.

barbara.demick@latimes.com

Ju-min Park in The Times' Seoul Bureau contributed to this report.