BEIJING - China formally protested Tuesday that three of its citizens were killed and a fourth wounded by North Korean border guards who opened fire Friday 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, on the other side of the bridge. The reports said they were on a boat on the river when they were shot.

"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 in Beijing.

The incident comes in the midst of a furor over the March 26 sinking of a South Korean warship that killed 46 sailors. At least publicly, China has not taken sides, infuriating 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. He applauded, however, China's protest over 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 long-standing 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 taking banned Chinese products, such as DVDs or mobile telephones, into North Korea.

The North Korean government is especially strict about the export of copper, which has in the past been looted from factories and 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," Kim said.