Toner said the US calls on all countries to use every available channel and means of influence to make clear to the North and its enablers that further provocations are unacceptable, and take steps to show there are consequences to its unlawful conduct.

He also urged the North to refrain from provocative actions and inflammatory rhetoric, and to make the strategic choice to fulfill its international obligations and commitments and return to serious denuclearization talks.

"Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad," Toner said. "We remain prepared, and will continue to take steps to increase our readiness to defend ourselves and our allies from attack, and are prepared to use the full range of capabilities at our disposal against this growing threat."

Earlier, South Korea's military said the North fired four ballistic missiles into the East Sea.

The launches took place from an area near the North's Dongchang-ri long-range missile site that Pyongyang has used to fire long-range rockets, raising fears that the communist nation might have tested an intercontinental ballistic missile.

But the South said such a possibility is low.

The missile launches are seen as a show of force in response to the ongoing annual joint military exercises between South Korea and the United States that the North has long denounced as a rehearsal for an invasion of the country.

The launches also came three weeks after the North test-fired a new intermediate-range ballistic missile powered by solid fuel.

Weapons experts say solid-fuel missiles pose greater threats as they require less launch preparation time than liquid-fueled rockets, and can be fired from mobile launchers, which are easy to move around.

The latest provocations could have an impact on the North Korea policy that the administration of US President Donald Trump is putting together. News reports said that the Trump administration is considering a wide range of options, including pre-emptive strikes.

North Korean leader Kim Jong-un said in his New Year's Day address that the North entered the final stage of preparations to test-fire an ICBM, an apparent threat that he is close to perfecting capabilities to strike the continental US with a nuclear missile.

In response, Trump said the North's development of such a missile "won't happen." (Yonhap)