North Korea says long-range cruise missiles were successfully test fired over the weekend, prompting a new investigation into the country's arsenal.
South Korea is analysing the weekend's launches with the help of US intelligence, after the North's state-controlled media group KCNA claimed targets were hit from 932 miles away.
If accurate, this range would indicate an ability to hit much of Japan, a strong ally of the US.
Saturday and Sunday marked the first projectile tests for months from North Korea, which the country hailed a "strategic weapon of great significance".
It builds on the intent of leader Kim Jong Un to build military capability while nuclear negotiations with the US continue to stall.
There is concern North Korea is exploiting a loophole in rules set out by the United Nations Security Council.
Ballistic missile launches, which follow a predetermined ballistic flight path once the propellant fuel is used, are prohibited under a resolution alongside nuclear tests by the North.
The launch of cruise missiles, self-propelled from start to finish and normally travelling at lower altitudes, is not explicitly banned in the North by the UN.
Ankit Panda, Senior Fellow, Carnegie Endowment for International Peace, is suspicious of the new missile capability and whether it has the potential to carry nuclear payloads.
The reported weapon "would be the first cruise missile system in North Korea with a 'strategic' role," he wrote on Twitter.
He added that "analysts know what North Korea means when it says 'strategic', but it's a political matter to convince all at the UNSC (and especially China and Russia) that the term unambiguously implies WMD (it does)".
US Indo-Pacific Command said the action demonstrates North Korea's "focus on developing its military programme and the threats that poses to its neighbours and the international community".
"The US commitment to the defense of the Republic of Korea and Japan remains ironclad," it said.
KCNA said the missiles tested over the weekend travelled for 126 minutes "along an oval and pattern-8 flight orbits" above North Korean land and waters before hitting their targets.
It added: "The test launches showed that the technical indices such as the thrust power of the newly developed turbine-blast engine, the missiles' navigation control and the end guided hit accuracy by the combined guided mode met the requirements of designs.
"In all, the efficiency and practicality of the weapon system operation was confirmed to be excellent."
North Korea ended a year-long pause in ballistic tests in March by firing two short-range ballistic missiles into the sea.
Talks between the US and North Korea have stalled since the collapse of a summit between then-President Donald Trump and Mr Kim in 2019, when the Americans rejected the North's demand for major sanctions relief in exchange for a partial surrender of its nuclear capabilities.
Mr Kim's government has so far rejected the Biden administration's overtures for dialogue, demanding that Washington abandon its "hostile" policies first.