Since leader Kim Jong Un avowed his commitment to military modernisation at a key party speech last month, the nuclear-armed country has conducted a string of weapons tests, including of hypersonic missiles.

Washington hit Pyongyang with fresh sanctions last week and North Korea responded by doubling down on testing, asserting its "legitimate right" to self-defence.

The most recent weapons test involved two "tactical guided missiles" that "precisely hit an island target in the East Sea of Korea," state news agency KCNA said Tuesday.

The launches "confirmed the accuracy, security and efficiency of the operation of the weapon system under production," it added.

South Korea's military earlier said two suspected "short-range ballistic missiles" were fired east from near Pyongyang Monday, flying 380 kilometres (about 240 miles) at an altitude of 42 km.

Japan also detected the test, with Defence Minister Nobuo Kishi warning the string of recent launches indicates North Korea "is trying to improve its technology and operational capability".

The launches come at a delicate time in the region, with North Korea's sole major ally China set to host the Winter Olympics next month and South Korea gearing up for a presidential election in March.

- 'Narrow the gap' -

Pyongyang is using the new US sanctions as an excuse to proceed with pre-planned conventional weapons tests, an analyst said, as the North seeks to narrow the gap with South Korea's well-equipped military.

"It lags far behind on that point in its rivalry with Seoul," said Cheong Seong-chang of the Center for North Korea Studies at the Sejong Institute.

"Though it is nuclear-armed, it is not in a position to use them unless it's attacked first and it can only use conventional weapons for possible military conflict along the border areas."

For now, talks with the United States are "off the table" as Pyongyang won't respond until Seoul and Washington freeze joint drills -- something Washington has said is not up for negotiation, he added.

The United States called on North Korea Monday to "cease its unlawful and destabilizing activities".

The US special representative on North Korea, Sung Kim, "expressed concern" about the missile launches and urged Pyongyang to return to dialogue "without preconditions", the State Department said.

Even as it flexes its military muscles, the impoverished country, reeling economically from a self-imposed coronavirus blockade, quietly restarted cross-border trade with China this weekend.

A freight train from North Korea arrived at the Chinese border city of Dandong for the first time since early 2020 on Sunday.

"Freight trains carrying goods between Dandong and the DPRK have resumed operation. This work will be conducted in accordance with pandemic prevention measures," Chinese foreign ministry spokesman Zhao Lijian said at a routine briefing Monday.