The reclusive communist country has conducted six sanctions-busting launches in less than two weeks, the latest coming Thursday with the firing of a pair of ballistic missiles.

On Tuesday, it fired an intermediate-range ballistic missile over Japan, prompting an alert for people in affected areas underneath to take cover.

"The missile test launch by the DPRK is a regular and planned self-defensive step for defending the country's security and the regional peace from the US direct military threats that have lasted for more than half a century," North Korea's civil aviation agency said, without specifying which launch, according to state-run news agency KCNA.

The government agency issued the statement after the International Civil Aviation Organization, holding its annual assembly in Montreal, on Friday condemned North Korea's missile tests of recent months and called them a danger to civil aviation.

The Korean agency said the ICAO's resolution was "a political provocation of the US and its vassal forces aimed to infringe upon the sovereignty of the DPRK" -- the initials of North Korea's official name.

Seoul, Tokyo and Washington have ramped up joint military drills in recent weeks, and carried out more exercises Thursday involving a US Navy destroyer from the USS Ronald Reagan aircraft carrier's strike group.

The launches are part of a record year of weapons tests by isolated North Korea, which leader Kim Jong Un has declared an "irreversible" nuclear power, effectively ending the possibility of denuclearisation talks.

The United States on Friday imposed sanctions on firms and individuals in Asia accused of helping Pyongyang procure fuel in violation of UN sanctions.

Washington also held a trilateral call with Tokyo and Seoul and the three agreed they would "continue to closely coordinate their near- and longer-term responses, including with allies and UN partners", the US State Department said in a statement.

Pyongyang's latest missile launches "pose a grave threat to regional peace and security", it said.

North Korea on Saturday released a separate statement, saying it was "seriously approaching the extremely worrisome development of the present situation", referring to the nuclear-powered USS Ronald Reagan being part of the US-South Korea military drills this week.

The joint exercises by Seoul and Washington have always infuriated the North as Pyongyang considers them rehearsals for invasion.

The drills are "extremely provocative and threatening", KCNA said Saturday, and the recent involvement of the USS Ronald Reagan is "a sort of military bluffing" against Pyongyang's "righteous reaction" to protect itself from US threats, it added.

Analysts say Pyongyang has seized the opportunity of stalemate at the United Nations to conduct ever more provocative weapons tests.

Officials in Seoul and Washington have been warning for months that Pyongyang will also conduct another nuclear test, likely after China's Communist Party Congress later this month.