The South Korean Embassy in Beijing was hit by a metal ball believed to have been fired from an air gun, officials said Wednesday, as diplomatic tensions grow over the killing of a Korean Coast Guard officer by a Chinese fisherman. An investigation by Chinese police has been under way since one of the embassy\'s bullet-proof windows was cracked on Tuesday afternoon by the ballbearing-shaped \"iron bead\" that was suspected to be fired from an air gun, officials said. No one was hurt. The incident is believed to have happened between 12:30 p.m. and 1:30 p.m. and the Korean embassy has asked the Chinese authorities to beef up security at the embassy, officials said.\"Our embassy in Beijing requested the Chinese authorities to protect the embassy building and carry out a swift investigation and relevant measures,\" said an official at Seoul\'s foreign ministry. It was not clear whether the embassy incident was related to the stabbing death of the Korean officer and no one reported hearing a gunshot, the official said on the condition of anonymity.It is the first time since the two nations established formal diplomatic relations in 1992 that the Korean embassy has experienced an apparent attack, the official said. In South Korea, public anger continues to run high after the 41-year-old officer, Lee Cheong-ho, was fatally stabbed to death on Monday by a Chinese fishing vessel captain during a raid on the ship for illegal fishing in Korean waters.The deadly incident, which occurred at a time of increasing violence by Chinese fishermen caught illegally fishing in South Korea\'s exclusive economic zone, threatens to develop into a diplomatic thorn between the two nations, with Seoul demanding an apology from Beijing. President Lee Myung-bak called for \"strong\" measures to ensure the safety of Coast Guard officers. South Korea delivered a strong protest at the killing by summoning the Chinese ambassador, but local media and politicians continued criticism of Beijing for failing to apologize for the fatal stabbing.On Tuesday, China\'s foreign ministry expressed \"regrets\" over the killing, calling it \"an unfortunate incident.\"