The North's elevated restrictions on movement's in the wake of the COVID-19 outbreak could cause a further strain on the country's economic difficulties, some experts say.
Kim defended his accelerating weapons development as a rightful exercise of sovereign rights to self-defence.
The flight came a day after the allies fired eight surface-to-surface missiles into South Korea’s eastern waters.
North Korean leader Kim Jong Un could further escalate his brinkmanship by conducting what would be the country’s seventh nuclear test since 2006.
Around 219,030 North Koreans with fevers were identified in the 24 hours through 6 p.m. Friday, the fifth straight daily increase of around 200,000.
State media reports said Kim and other senior North Korean officials are donating their private reserve medicines to support the country's anti-pandemic fight.
Speaking to thousands of troops and spectators mobilised for the event, Kim vowed to develop his nuclear forces at the "fastest possible speed" and threatened to use them if provoked.
Experts say North Korea's announcement of the letters, which came as Kim possibly prepares for a nuclear test and other major provocations, is aimed at dividing public opinion in South Korea.
That test was the culmination of a record-breaking blitz of sanctions-busting launches this year and signalled an end to a self-imposed moratorium on long-range and nuclear tests.
On Friday, Pyongyang residents bowed and lay bouquets of flowers near the bronze statues of Kim Il Sung and Kim Jong Il.
North Korea announced the completion of the apartment complex ahead of the 110th anniversary Friday of the birth of founding leader Kim Il Sung -- Kim Jong Un's grandfather.
Experts say Kim is trying to boost his support from elite North Koreas while the country deals with pandemic hardships and a diplomacy stalemate.
North Korea said Monday leader Kim Jong Un has vowed to develop more powerful means of attack, days after the country's first intercontinental ballistic missile launch in more than four years.
Though North Korea’s neighbors detected two ballistic launches last week, they later said it was testing cameras and other systems to be installed on a spy satellite.
North Korea has used the pause in talks to further expand its military capabilities, including nine rounds of missile launches in 2022 alone.