North Korea fired two medium-range ballistic missiles into the East Sea on Sunday morning from the site it claimed to have tested a high-thrust solid-fuel engine three days ago, apparently used for developing newer and more advanced intercontinental ballistic missiles.

The National Security Office vowed to "take all possible measures" to protect the people from North Korea’s provocations and to boost security alliance with the US and Japan to deter growing threats from Pyongyang.

The Joint Chiefs of Staff said the two ballistic missiles were fired at a steep angle from Tongchang County, North Pyongan Province, between 11:13 a.m. and 12:05 p.m. and flew some 500 kilometers before falling to the East Sea.

Tongchang County is where North Korea claimed on Friday to have succeeded in testing a high-thrust solid-fuel engine that could be used for developing more advanced intercontinental ballistic missiles.

Until now, North Korea has only launched ICBMs that use liquid fuel. But the test of a high-thrust solid-fuel engine could raise the possibility of developing ICBMs that use solid fuel. Solid-fuel ICBMs do not require refueling time so they are known to be able to hit the US mainland by avoiding prior detection and interception by South Korea and the US with a surprise launch.

The NSC attendees "paid attention to North Korea’s tests of solid fuel propulsion engines" and deplored the behavior of the Kim Jong-un regime, which continues missile provocations regardless of the people suffering from cold and hunger, according to Lee Jae-myung, a vice spokesperson of the presidential office, in the afternoon press briefing.

The JSC also condemned Pyongyang's missile launch, saying that it is a major provocation that harms peace and stability on the Korean Peninsula as well as the international community. "We strongly condemn it as a clear violation of the UN Security Council resolutions and urge an immediate halt."

Kim Gunn, the South Korean special representative for Korean Peninsula peace and security affairs, held talks with his US and Japanese counterparts to discuss countermeasures.

They reiterated the importance of uniting the international community and responding decisively to North Korea's illegal provocations, which directly violated a number of UN Security Council resolutions. To this end, they agreed to continue strengthening bilateral and trilateral communication and coordination.

Senior Vice Defense Minister Toshiro Ino told reporters that North Korea's repeated ballistic missile launches are absolutely unacceptable and Japan lodged a protest with Pyongyang via its embassy in Beijing.

Japan's Defense Ministry said both missiles have apparently landed outside of Japan's exclusive economic zone, according to Japanese public broadcaster NHK's report. "There have been no reports of damage to Japanese vessels and aircraft so far,” the Japanese Defense Ministry added.

The ballistic missile launch is seen as Pyongyang's response to the United Nations' adoption of a resolution on North Korean human rights for the 18th consecutive year and Japan's adoption of a security strategy to secure the ability to counterattack the enemy base, according to Kim Yong-hyun, a professor of North Korean studies at Dongguk University.

The latest provocation by North Korea is the first in 12 days following multiple rocket launches on Dec. 6. At the time, North Korea fired around 100 artillery shells toward the inter-Korean maritime buffer zone in a tit-for-tat action against South Korea and US live-fire drills staged near the border.

It also came about a month after it launched the Hwasong-17, an intercontinental ballistic missile, from the Sunan area of Pyongyang toward the east sea on Nov. 18.

North Korea fired 64 ballistic missiles 36 times this year and three cruise missiles.

Lee Sang-min, head of the Korea Institute for Defense Analyses’ North Korean military research division, analyzed that North Korea had spent between $340 million and $530 million in launching 61 ballistic missiles by November of this year.

The ruling and opposition parties condemned North Korea's provocations.

Park Jung-ha, a senior spokesperson of the ruling People Power Party, said in a commentary: “The alliance that protects freedom and peace, including South Korea, strongly condemns the Kim Jong-un regime's behavior and will surely hold it accountable for its hostile actions.”

Main opposition Democratic Party of Korea spokesperson Park Sung-joon said in a written briefing: "We strongly condemn (North Korea’s provocations) for continuing to increase military power and heightening tensions in Northeast Asia."