North Koreans on Sunday marked the sixth anniversary of the death of Kim Jong Il, the former leader of the country and father of current leader Kim Jong Un.

The commemoration came amid heightened tensions with the U.S. over concerns about North Korea's nuclear weapons program.

Bearing flowers, thousands of somber-faced North Koreans were photographed bowing in front of statues and portraits of their leaders in freezing weather.

As sullen, recorded music played, the mourners marched up Pyongyang's Mansu Hill and placed flowers at the feet of two giant bronze statues of Kim Jong Il and national founder Kim Il Sung, who is North Korea's "eternal president" and Kim Jong Un's grandfather.

Kim Jong Il died at age 69 on Dec. 17, 2011, reportedly of a heart attack. Kim was known to have had health problems in the years before his death, including a stroke in 2008.

North Koreans are expected to avoid drinking, entertainment and inappropriate displays of enjoyment on the day before and the day of the anniversary.

At midnight, senior ruling party officials visited a mausoleum on the outskirts of the city where the elder Kims lie in state, according to state-run media reports.

Experts have warned that North Korea has shown a tendency to conduct missile tests to mark major national events.

In April, tens of thousands of North Korean soldiers marched in a parade to mark the 105th anniversary of the birth of Kim Il Sung.

The next day, the country conducted a failed missile test from Sinpo, the U.S. military said.

And last year, North Korea conducted two missile tests days after "Party Foundation Day" on Oct. 10, a public holiday in the country.

North Korea claimed in late November to have tested an intercontinental ballistic missile capable of striking the U.S. mainland. But Secretary of Defense James Mattis cast doubt on the claim Friday, saying the U.S. was still assessing the situation.

The Associated Press contributed to this report.