North Korea on Tuesday fired what are believed to be ballistic missiles toward the sea in an apparent show of force, according to officials in neighboring countries, amid rising tensions with South Korea ahead of a key political gathering in Pyongyang.

Japan’s Prime Minister’s Office and Defense Ministry reported the launch of possible ballistic missiles, while South Korea’s Defense Ministry said it detected at least one unidentified projectile launched off North Korea’s eastern coast. Details regarding the range or trajectory of the weapons were not immediately disclosed.

The launch followed recent threats from North Korea to retaliate over alleged South Korean surveillance drone flights across the border, reportedly occurring earlier this month and in September. South Korea has denied conducting drone operations during the periods mentioned and said it is investigating whether civilian drones may have been involved.

Experts say the drone allegations are likely part of Pyongyang’s efforts to stoke anti-South Korean sentiment ahead of the ruling Workers’ Party congress, expected to be held in late January or February. During the congress, North Korea may formally incorporate leader Kim Jong Un’s declaration of a hostile “two-state” approach toward the Korean Peninsula into the party constitution — a move not seen in the past five years.

Earlier this month, North Korea announced test flights of hypersonic missiles, which were reportedly observed by Kim Jong Un, who emphasized strengthening the country’s nuclear deterrence, according to state media.

Read More: North Korea claims progress in hypersonic missile program amid regional tensions

In December, the North said it had test-fired long-range strategic cruise missiles and new anti-aircraft missiles, and released images suggesting the construction of its first nuclear-powered submarine.

Analysts believe North Korea is showcasing its progress in weapons development ahead of the upcoming party congress.