(Reuters) - North Korean leader Kim Jong Un ordered the execution of 15 senior officials this year as punishment for challenging his authority, South Korea's spy agency told a closed-door parliament meeting on Wednesday.

A vice minister for forestry was one of the officials executed for complaining about a state policy, a member of parliament's intelligence committee, Shin Kyung-min, quoted an unnamed National Intelligence Service official as saying.

"Excuses or reasoning doesn't work for Kim Jong Un, and his style of rule is to push through everything, and if there's any objection, he takes that as a challenge to authority and comes back with execution as a showcase," Shin said.

"In the four months this year, fifteen senior officials are said to have been executed," Shin cited the intelligence official as saying, according to his office.

Shin added four members of the Unhasu Orchestra, where Kim's wife, Ri Sol Ju, previously performed as a singer, were executed by firing squad in March for espionage, without elaborating.

In 2013, Kim purged and executed his uncle, Jang Song Thaek, once considered the second most powerful man in Pyongyang's leadership circle, for corruption and committing crimes damaging to the economy, along with a group of officials close to him.

Kim has also reshuffled close aides and senior officials repeatedly since taking office.

South Korea's spy agency also expected Kim to travel to Moscow this month to attend an event marking the end of World War Two in Europe, although there was no independent confirmation of the plan, Shin said after the spy agency briefing.

North Korea has not booked a hotel in Moscow for Kim's stay, but the country's embassy was equipped to accommodate its leader, Shin said, quoting the spy agency official.

The visit would be Kim's first overseas trip since he took power in 2011 after the death of his father.

Russia has said Kim would attend the May 9 event marking the 70th anniversary of the war's end in Europe, although officials in Seoul have cautioned that there was no official confirmation from the North.

Some analysts have questioned whether Kim, believed to be in his early 30s, would choose for his first overseas visit an event where he would share the stage with several leaders and have less control over proceedings than in a two-way summit.

South Korean President Park Geun-hye has decided not to attend the function. U.S. President Barack Obama and many European leaders are staying away, but Chinese President Xi Jinping and the heads of many former Soviet republics are expected to attend.