North Korean leader Kim Jong-un has ordered the execution of 15 people this year, including several top officials, South Korean intelligence says.
The agency told a parliamentary meeting on Wednesday that they were executed by firing squad on spying charges.
Those killed include two vice ministers who challenged Mr Kim over his policies and members of an orchestra, intelligence officials said.
Mr Kim purged and executed his once-powerful uncle for treachery in 2013.
South Korean politicians were told that one of the officials killed was a forestry minister who had complained about North Korea's forestation plan, Yonhap news agency reported.
"Excuses or reasoning doesn't work for Kim Jong-un, and his style of rule is to push through everything," said Shin Kyung-min, a member of South Korea's parliamentary intelligence committee, quoting an intelligence official.
Analysis: Stephen Evans, BBC News, Seoul
From outside Pyongyang, it's impossible to know exactly what's going on in Kim Jong-un's inner circle.
The South Korean spy agency may have informants but it also relies on hit-or-miss methods like analysing photographs in the North Korean state-controlled media.
We do know that Mr Kim's uncle, Chang Song-thaek, was executed in 2013 because North Korean media reported it.
Last year, six important officials suddenly vanished from public view and South Korean media reported they had been executed.
If it is true that 15 senior people have been executed in the four months of this year - a rate of one a week - it would indicate that the regime is getting tougher.
Kim Jong-un has been in power for just over three years. He is as secure in his job as any dictator can be, but does he feel secure?
Mr Shin's office told Reuters news agency that the unnamed official had added that the executions were a "showcase" in response to what Mr Kim saw as a challenge to his authority.
Four members of North Korea's Unhasu Orchestra are also believed to have been executed in March.
Reports say that Mr Kim's wife, Ri Sol-ju was a singer in the orchestra before her marriage.
South Korean politician Lee Cheol-woo told Yonhap that the head of the orchestra had been executed, possibly for leaking family secrets.
There has been no confirmation from North Korea about the executions but Mr Kim has purged his opponents before.
In 2013, Mr Kim put his uncle, Chang Song-thaek, to death, along with a group of officials who had been close to him.
Mr Chang was seen as a mentor to Kim Jong-un during the leadership transition from his father Kim Jong-il in 2011.