The two missiles flew some 500 kilometres and reached a maximum altitude of around 550 kilometres before landing in the Sea of Japan.
North Korea fired two ballistic missiles, Seoul's military has said, days after Pyongyang announced a successful test of a solid-fuel motor for a new weapons system.
South Korea detected two ballistic missiles that had been fired from the Tongchang-ri area in North Pyongan province, the country's Joint Chiefs of Staff said on Sunday.
The missiles were fired from 11:13 am local (0213 GMT) to 12:05 pm into the East Sea, it said, referring to the body of water also known as the Sea of Japan.
"Our military has strengthened surveillance and vigilance while closely cooperating with the United States and maintaining a full readiness posture," the JCS added in a statement.
The missile flew some 500 kilometres and reached a maximum altitude of around 550 kilometres, according to Japan's defence ministry.
"It threatens the peace and security of our country, this region, and the international community, and it is absolutely unacceptable," said senior vice defence minister Toshiro Ino.
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New-type strategic weapon system
Military tensions on the Korean peninsula have risen sharply this year as Pyongyang has carried out an unprecedented blitz of weapons tests, including the launch of its most advanced intercontinental ballistic missile ever last month.
Sunday's launch came days after North Korea tested a "high-thrust solid-fuel motor", with state media describing it as an important test "for the development of another new-type strategic weapon system".
Despite heavy international sanctions over its weapons programmes, Pyongyang has built up an arsenal of intercontinental ballistic missiles (ICBMs).
All its known ICBMs are liquid-fuelled, however, and North Korean leader Kim Jong Un has placed strategic priority on developing solid-fuel engines for more advanced missiles.
Kim said this year that he wants North Korea to have the world's most powerful nuclear force, and declared his country an "irreversible" nuclear state.
The wishlist he revealed last year included solid-fuel ICBMs that could be launched from land or submarines.
The latest motor test was a step towards that goal, but it is not clear how far North Korea has come in the development of such a missile, analysts said.
READ MORE: North Korea performs key test to build more advanced ICBM
Key DPRK party meeting
The isolated country's policy direction for next year will be laid out at a key party meeting later this month, and the official Korean Central News Agency earlier reported Kim saying that 2023 would be a "historic year".
In past years, Kim had delivered a speech every January 1, but has recently dropped the tradition in favour of making announcements at the year-end plenary meeting.
In his most recent address, which was released last New Year's Day, Kim focused on domestic affairs.
Experts say while Kim refrained from directly addressing the United States last year, he could change his tone this time around.
The United States and South Korea have warned for months that North Korea is preparing to conduct its seventh nuclear test.
North Korea is under multiple UN Security Council sanctions over its nuclear and missile activity since 2006.
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