The Japanese government declined to comment on U.S. media reports about North Korean leader Kim Jong-un allegedly being in "grave danger" following surgery.

At a press briefing on Tuesday, Chief Cabinet Secretary Yoshihide Suga said he wasn't going to comment on each and every report, adding that Japan plans to collect and analyze related intelligence through close cooperation with the United States and other countries.

Acknowledging that he is aware of such reports, Suga reiterated that the country is collecting information on North Korea, a matter of grave concern.

Citing a U.S. official, CNN reported earlier that Washington is monitoring intelligence that Kim is in "grave danger" after surgery.

Quoting sources inside North Korea, South Korea's web-based media outlet Daily NK reported that Kim underwent a "cardiovascular surgical procedure" on April 12 and is currently recovering.