Lee Myung-bak said North Korea's actions were "not desirable," according to South Korean pool reports.
Lee was personally targeted in a North Korean propaganda missive earlier this week that labeled him a "traitor" and warned that the South would face unspecified consequences for Lee's pro-U.S. policy.
The North has also threatened to turn the South to "ashes" in a pre-emptive strike, responding to remarks by South Korea's top military officer that Seoul could target suspected North Korean nuclear sites if there were signs of a pending atomic attack.
Lee said Thursday that the officer's comments "were a natural answer" in response to lawmakers' questions at a parliamentary hearing. He added that the North should refrain from "taking issue with remarks that don't have other meanings."
In the past week, North Korea has also test-fired missiles and expelled South Korean officials from a jointly run industrial zone, apparently in angry response to the tougher policies of South Korea's new conservative government.
Lee, who has criticized his two liberal predecessors for giving unconditional aid to North Korea, has said he would link assistance to progress in the North's nuclear disarmament.
