U.S. and North Korean national flags are seen at the Capella Hotel on Sentosa island in Singapore June 12, 2018.
U.S. and North Korean national flags are seen at the Capella Hotel on Sentosa island in Singapore June 12, 2018. Reuters / Jonathan Ernst

U.S. forces stationed in South Korea said on Tuesday they have enhanced the intensity of exercises for their Patriot missile air defence system amid signs that North Korea might soon conduct another long-range missile test.

Tension on the Korean Peninsula has been growing amid speculation North Korea could test its largest intercontinental ballistic missile (ICBM) as soon as this week at full range, after threatening to break a self-imposed 2017 moratorium on long-range missile and nuclear testing.

U.S. Forces Korea (USFK) said its air defence artillery brigade stationed in the Osan air base has recently increased the intensity of its certification exercise to demonstrate its capabilities following North Korea's recent missile tests.

"DPRK's significant increase in its missile testing activity undermines peace, security and destabilises the Northeast Asia region," USFK said, using the acronym of North Korea's official name, the Democratic People's Republic of Korea.

"While this type of training is routinely conducted by U.S. Patriot batteries ... its increased intensity of its certification underscores the seriousness USFK takes against the DPRK's recent missile launch behaviour," USFK said.

A spokesman for South Korea's defence ministry said it was also closely monitoring North Korea's movements and maintaining a robust combined readiness posture with the U.S. military.

On Friday, Washington and Seoul said in a rare joint announcement that Pyongyang had used its largest-ever ICBM in two recent launches, in the guise of satellite launch preparations.

But the launches did not demonstrate the missile's full range, and analysts said the North might have used only one stage of the missile or adjusted its fuel volume to fly at lower altitudes.

The missile system, the Hwasong-17, was unveiled at a military parade in 2020 and reappeared at a defence exhibition in October 2021.

USFK also said the U.S. Indo-Pacific Command has recently enhanced its ballistic missile defence directive.

The U.S. military in Japan on Tuesday flew its RC-135S Cobra Ball spy aircraft over the Korean peninsula, a day after sending an RC-135V Rivet Joint reconnaissance plane, the Yonhap news agency said, citing a flight-tracking website.