The test came 77 days after Pyongyang's short-range missile launch in early May.

South Korea's National Security Council tentatively assessed them as being "a new kind of short-range ballistic missile," the presidential office Cheong Wa Dae said. A final assessment would come after a detailed analysis with the United States, it added.

The top security body expressed "strong regret" over the provocation, saying it does not help efforts to ease military tensions on the peninsula.

The missiles were presumed to have been fired from a transporter erector launcher.

"We believe that (North Korean leader) Kim Jong-un has recently stayed in the region, and summertime military drills are now under way in the North," a JCS officer noted.

It is too early to say whether the two missiles were the same type and whether they were same as the short-range missiles that the North test-fired twice in May, according to the officer.

"Our military is closely monitoring the situation in case of additional launches while maintaining a readiness posture," the JCS said.

Experts have said Thursday's launches appear to have involved the North's version of Russia's Iskander, a short-range, ground-to-ground ballistic missile known as KN-23, as it test-fired in May.

On May 4, the North launched a fusillade of projectiles, which involved "a new type of tactical guided weapon" and 240-millimeter and 300-mm multiple rocket launcher systems. The projectiles flew about 70 km to 200 km, according to the JCS.

The North later said that they were "routine" and "self-defensive" drills that were not intended as provocations.

Five days later, it fired a barrage of projectiles, including two short-range missiles that flew 270 km and 420 km.

"This time, North Korea appears to have adjusted the weight of its warhead to make it fly farther," said Chang Young-keun, a professor of aerospace and mechanical engineering at Korea Aerospace University.

Since its first launch in May, North Korea was known to have improved the capabilities of the new weapon.

Iskander has several versions, and detailed specifications of Pyongyang's Iskander were not known. But the solid-fuel missiles can fly as far as 500 km to put the entire Korean Peninsula within their range.

Due to its relatively low peak altitude, Iskander could neutralize the advanced US anti-missile defense system, and it is nearly impossible to prevent their launches due to their mobility, according to experts.

Though Seoul and Washington have not confirmed the exact types of the missiles, Defense Minister Jeong Kyeong-doo said in early June that he believes the weapons North Korea fired in two rounds of tests in May were nearly the same type, though some differences have been spotted.

If they are confirmed to be ballistic missiles, North Korea will be in violation of UN Security Council resolutions that ban Pyongyang from conducting all kinds of ballistic missile launches.

In response to Thursday's launches, South Korea's defense ministry called on Pyongyang "to stop such acts that do not help efforts to ease military tensions on the Korean Peninsula" and vowed to ramp up surveillance in close cooperation with the US.

The foreign ministry also said it is in close consultations with the US and Japan in sharing up-to-date information and analysis on the missile firings.

South Korea will continue its diplomatic efforts for the resumption of denuclearization talks for a swift outcome and plans to discuss the matter with China and Russia, the ministry added.

The latest firings took place ahead of possible talks between Pyongyang and Washington on the North's nuclear weapons program, though the outlook remains uncertain as Pyongyang appears to make dialogue contingent on joint drills that Seoul and Washington plan to hold next month.

In June, the North Korean leader and US President Donald Trump agreed to resume their working-level talks within a few weeks, when they met at the inter-Korean border last month.

Negotiations on North Korea's nuclear weapons program have been stalled since February after the summit between Trump and Kim ended without results.

Pyongyang has reportedly not responded to Washington's offer for dialogue. Instead, it has also called the US and South Korea to cancel their combined military exercise slated for August, warning that the drill would affect ongoing efforts to resume its nuclear talks.

Despite such warnings, Seoul and Washington have said they are scheduled to launch the exercise as planned, stressing that the command post exercise is meant to test South Korea's operational capabilities for the envisioned transfer of the wartime operational control.

Thursday's firing followed North Korean leader Kim's recent field inspection of a newly built submarine. During the visit to an unspecified place, Kim called for the deployment of naval forces to boost his country's military capabilities, according to the North's Korean Central News Agency on Tuesday.

"The latest saber-rattling by North Korea, albeit in low intensity, appears to have multiple purposes: It is an overt expression of its complaints over the Seoul-Washington joint military drills and is part of efforts to put more pressure on the US so as to gain more concessions before the resumption of talks," said Kim Dong-yup, a professor at Kyungnam University's Far East Institute.

"Internally, the North is aiming to persuade its military who may be discontented with such peace efforts, while sending messages to its people that its military might remains strong," he added.

A diplomatic source here said, "More time seems to be needed before working-level talks resume, though we need more analysis of their intentions behind the latest military actions."

Adding to uncertainty, North Korean Foreign Minister Ri Yong-ho is reportedly unlikely to attend the ASEAN Regional Forum to be held in Thailand early next month.

North Korea test-fired the Hwasong-15 intercontinental ballistic missile in November 2017, and leader Kim has since declared a moratorium on nuclear and ICBM tests.

On Monday, Trump stressed North Korea's halt to testing nuclear weapons and long-range missiles, saying that he had "very positive correspondence" with North Korea.

"Again, there's no nuclear testing, there's no missile testing, there's no nothing. I think we will, yeah, at a certain point, when they're ready, we'll be meeting," Trump said. (Yonhap)