South Korea's presidential Blue House says the missile launches are "very worrisome" and unhelpful for efforts to improve inter-Korean relations amid protracted talks on denuclearisation.
North Korea fired what appeared to be two short-range missiles on Thursday, the South's military said, less than a week after its leader Kim Jong-un oversaw the test-firing of multiple rockets and a missile.
The launches came as US special envoy for North Korea Stephen Biegun was in the South Korean capital for talks with Foreign Minister Kang Kyung-wha and his counterpart nuclear envoy Lee Do-hoon.
The two missiles were fired from the north-west area of Kusong, in an easterly direction, the South's Joint Chiefs of Staff said in a statement.
They covered distances of 420 km (260 miles) and 270 km (168 miles) and reached an altitude of about 50 km (31 miles) before falling into the sea, they said.
Analysts said it was too soon to say exactly what kind of missiles they were.
"You don't know what missile it is just from how far it flew," said Yang Uk, a senior research fellow at the Korea Defence and Security Forum.
Such a range would probably exceed that of most of the rockets North Korea fired on Saturday from its east coast into the ocean, sharply raising tension at a time of stalemate in its talks with the United States.
"North Korea has returned to its classic escalation tactics from before," Yang added. "I believe they will keep escalating by using what appear to be short-range missiles, something that will not cause the US to react right away."
After Thursday's launch, South Korea's military said it had stepped up monitoring and security in case of another launch, and was working with the United States to get additional information on the missiles involved.
'Closely monitoring situation'
A spokeswoman for the Blue House said President Moon Jae-in's national security adviser Chung Eui-yong was closely monitoring the situation through a video conference with the military.
Reuters could not immediately reach US officials to seek comment.
Japan's defence ministry said it had not detected any ballistic missile in its territorial waters or exclusive economic zone and saw no direct impact on security.
North Korea has test-fired rockets over Japanese territory in the past.
Kusong is about 40 km (25 miles) north of the site of a medium-range Rodong missile base on North Korea's west coast, said the Center for Strategic International Studies, a Washington-based think tank.
'Regular and self-defensive'
On Saturday, North Korea also launched at least one short-range ballistic missile, which analysts said could travel up to 500 km (311 miles).
The launch, from an east coast area, was the North's first test of a ballistic missile since launching an intercontinental ballistic missile in November 2017.
This week, North Korea lashed out at the United States and South Korea, saying Saturday's tests were "regular and self-defensive" and rejecting the notion that they were provocative.
South Korea had responded to those tests by calling for an end to acts that escalate military tension.
At the end of 2017, North Korean leader Kim declared the country's nuclear force was complete and extended an olive branch to the South and the United States, holding two summits with US President Donald Trump and three with the South Korean leader.
But Kim's second summit with Trump, in Vietnam in February, broke down with the United States insisting the North give up its nuclear programme, which it had pursued for years in defiance of UN Security Council resolutions, and the North seeking sanctions relief.