South Korea said on Wednesday it had asked China for security guarantees at its Beijing embassy after the building was hit by a small projectile as tensions run high after the killing of a South Korean coastguard by a Chinese fisherman.

South Korea's Foreign Ministry said it was unclear what kind of weapon was used to fire the ballbearing-like shot that cracked one of the embassy's windows on Tuesday afternoon, but that no one was injured.

A ministry official, who asked not to be identified, told Reuters in Seoul the incident occurred between 12:30 p.m. and 1:30 p.m. on Tuesday and that an investigation was under way.

The Korean embassy has sent a letter to authorities requesting them to protect the mission and identify the reason for the incident, the official said.

In a report from Beijing, South Korea's Yonhap news quoted sources as saying the metal ball was likely fired from an air gun, as no one testified to hearing a gunshot.

An embassy official said it was difficult to comment on what kind of weapon, if any, had been used, but as there is another building between the glass and the street outside, a shot would have to have been fired from an upper floor of a building.

He declined to provide further details.

Chinese Foreign Ministry spokesman Liu Weimin said authorities were taking the embassy incident very seriously.

As far as we know, from the initial evaluation and from the onsite inspection, the South Korea Embassy had its window glass broken and has not been attacked, Liu told a news briefing.

At present, the Chinese side has taken relevant measures and will send more police and enhance the security patrols to increase the protection of the embassy.

South Korean President Lee Myung-bak called on Tuesday for strong measures to ensure the safety of coastguard officers cracking down on illegal Chinese fishermen amid a public uproar over the stabbing death of an officer on Monday.

Chinese boats are frequently caught illegally fishing in the Yellow Sea, an area rich with mackerel and crabs, and there have been a number of clashes with South Korean maritime police.

Beijing has expressed regret over the incident, but initially was unapologetic, calling on Seoul to protect the rights of the detained fishermen.

On Tuesday, a few hundred protesters staged a demonstration outside the Chinese embassy in Seoul. About 30 people attempted to enter the mission but were stopped by police, and a man crashed a car into a police bus at the embassy.

It is unclear if the embassy incident in Beijing was related to the protests.

Media reports in South Korea have criticized the government for not taking stiffer action against illegal Chinese fisherman after a similar incident three years ago when a coastguard officer was killed and six were injured.

Yonhap has reported that some 2,600 Chinese fishing boats have been caught illegally fishing in the South Korean exclusive economic zone since 2006 and nearly 800 Chinese fishermen have been arrested.