(Updates to add one of the wounded has died)
SEOUL, May 13 (Reuters) - A soldier in South Korea's reserves went on a shooting spree on Wednesday, killing a fellow soldier and injuring three comrades before fatally shooting himself, a military official said.
One of the soldiers injured in the attack during a mandatory training at a reserve forces site in Seoul later died.
The incident will spur questions over the country's rules on compulsory military service at a time when its military faces criticism of lax discipline in some units, leading to attacks on soldiers by colleagues suffering from psychological problems.
"The army is investigating the incident," said the military official, who said the reservist used his K-2 assault rifle in the attack. He was 24, said South Korean cable television network YTN.
South Korea requires two years of compulsory military service by all able-bodied men, since it is still technically at war with North Korea after their 1950-53 war ended in a truce and not a peace treaty.
They must then participate in annual military training as reservists for eight years.
South Korea's military has about 630,000 soldiers on active duty - more than 300,000 of them conscripts - to deter aggression from North Korea, one of the world's most militarised states.
But the country's declining birth rate and small families have prompted concerns that its armed forces have become too slow to deal with social changes.
Rights group Amnesty International on Wednesday urged South Korea to end the imprisonment of hundreds of young men who object to military service on grounds of conscience.
The most recent outcry concerned continuing physical and emotional abuse that led to the death of a conscript at the hands of his barrack mates last year.
Last year a conscript soldier threw a grenade and shot dead five comrades near the border with North Korea. (Reporting by Ju-min Park; Additional reporting by James Pearson; Editing by Tom Heneghan)