South Korea expressed displeasure Wednesday with North Korea's verbal attack on Lee Wan-koo, the nominee to become the South's prime minister.

The unification ministry said the ongoing confirmation hearing on Lee is an internal matter.

"It's regrettable that North Korea criticizes a nominee for (South Korea's) official position," ministry spokesman Lim Byeong-cheol said at a press briefing.

The two Koreas have already agreed to discontinue criticism and slandering each other and not to intervene in the other side's internal affairs, he pointed out.

In its main newspaper, the Rodong Sinmum, the North on Wednesday labeled Lee as a "boss of corruption."

It cited a number of local media reports on Lee, former floor leader of the ruling Saenuri Party, including his suspected real estate speculation and an alleged attempt to avoid mandatory military service by his son.

The South's ministry also urged the North to stop criticizing former President Lee Myung-bak by his name. Lee has come under renewed attack from the North since publicizing a memoir on his 2008-2013 presidency early this month. He said the North proposed inter-Korean summits talks several times and demanded huge economic reward in exchange.

"Such a criticism by North Korea on a former South Korean head of state is not desirable for South-North relations," Lim said. (Yonhap)