File photo shows people watching a TV showing file footage of North Korea's missile launch during a news program at the Seoul Railway Station in Seoul, South Korea. Image Credit: AP

Washington: The United States on Friday slapped sanctions on three companies and two people based in the Marshall Islands, Singapore and Taiwan for facilitating illegal oil sales to North Korea.

The move follows an escalating series of missile tests by Pyongyang, including the provocative launch of an intermediate-range ballistic missile over Japan.

"The United States is sending a clear message that we will continue to take actions against those who support the development and sustainment of the DPRK's military and weapons arsenal," Secretary of State Antony Blinken said in a statement, using the acronym for North Korea's official name.

According to the US Treasury Department, Marshall Islands-based New Eastern Shipping Co Ltd owned a ship called the Courageous - which was also known as the Sea Prima - that illegally delivered refined petroleum to a North Korean ship off the country's southwestern coast.

The delivery violates UN Security Council resolutions that prohibit ship-to-ship transfers with North Korean vessels and that cap the country's refined petroleum imports, Blinken said.

The ship "engaged in deceptive shipping practices," such as turning off identification systems and working at night, to "obfuscate its true destination or origin," the Treasury said in its own statement.

North Korea has launched six ballistic missiles - including over Japan - in less than two weeks, saying the sanctions-busting displays were necessary countermeasures against joint military drills by the United States and South Korea.

Friday's sanctions also target Singapore-based Kwek Kee Seng and Taiwan-based Chen Shih Huan, who were involved in the operation of the targeted ship, and two other companies run by Kwek.

The measures restrict Kwek's, Chen's and the companies' financial moves in the United States or with US entities.

North Korea defends missile launch as defense against US

North Korea on Saturday defended its recent flurry of missile tests as a legitimate defense against what it called US "military threats."

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

On Tuesday, the North fired an intermediate-range ballistic missile over Japan, prompting a call 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.

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.

Analysts say Pyongyang has seized the opportunity of stalemate at the UN 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 Party Congress on October 16.