North Korean leader Kim Jong-un has noticeably increased military-related public activity this year in the latest expression of the North's jitters over tougher U.N. sanctions, Seoul officials said Thursday.

The North's leader conducted the so-called field guidance in the military and defense sectors 16 times in the Jan. 1-March 18 period, accounting for 62 percent of his total public activities this year, according to South Korea's unification ministry.

The tally marked a 20 percent increase from the same period last year, it added.

The United Nations Security Council (UNSC) slapped tougher sanctions on North Korea in early March in response to the North's nuclear test in January and long-range rocket launch in February.

"An increase in Kim's military-related public activity appears to reflect a sense of crisis, which North Korea has felt since the tougher U.N. sanctions and ongoing joint military drills between Seoul and Washington," the ministry said in a statement.

South Korea said that the North's leader has continued to make aggressive and provocative acts since the UNSC adopted a fresh resolution punishing the North for its provocations.

The North's military had conducted defensive military drills before the sanctions came out. But the North has recently ratcheted up threats in a fit of anger against Seoul and Washington, vowing to carry out pre-emptive strikes.

The communist country said Wednesday that it is ready to turn President Park Geun-hye and her office into "a sea of flames and ashes," with its large-caliber multiple rocket launching system.

The North also said that it has successfully conducted a solid-fuel rocket test and has acquired the re-entry technology that is a key to hitting a far away target with a nuclear-armed ballistic missile.

South Korea and Washington rejected the North's claims, saying that based on their analysis, the North has yet to make nuclear warheads small enough to be amounted on ballistic missiles. They also raised doubts about the North's claims of the re-entry technology.

Meanwhile, the North's leader has significantly reduced his public activity on the economic front despite the urgency to produce accomplishments ahead of the ruling party's congress slated for May, the ministry said.

In the first quarter, Kim's on-site inspections of economy-related sectors accounted for 23 percent of the total, down about 20 percent from a year ago, it said.

The Workers' Party of Korea is scheduled to hold its first party convention in more than three decades in May, pushing North Koreans to work hard in the "70-day campaign of loyalty." (Yonhap)