SEOUL (THE KOREA HERALD/ASIA NEWS NETWORK) - Amid concern that North Korea might conduct another intercontinental ballistic missile (ICBM) test, tension is running high on the Korean Peninsula, with experts warning that Pyongyang may be preparing to launch an upgraded version of a submarine-launched ballistic missile (SLBM).

Citing a United States defence official, CNN reported on Thursday (July 27) that the North conducted a missile ejection test on Tuesday at a naval shipyard in Sinpo, South Hamkyong Province, where North Korea is thought to have repositioned submarines and barges used for an SLBM test in August last year.

The lift-off was designed to test the "cold-launch" system, a critical component of an SLBM and a technology used for the launch of the SLBM Pukguksong-1 in August last year (2016) and its land-based version Pukguksong-2 in February this year.

It was the second such test this month and the third this year, the officials added.

"We believe North Korea has been preparing for the launch of the Pukguksong-3", the latest variant of Pukguksong-type missiles, which were presumed to be enclosed in a canister during the North's military parade on April 15, said submarine expert Moon Geun Shik at the Korea Defence and Security Forum,

"Compared to the Pukguksong-1 and Pukguksong-2, the Pukguksong-3 is considered to require more fuel. We believe the North would upgrade the missile by applying its advanced technology for the warhead and re-entry system."

Since the successful launch of the Pukguksong-1 last August, North Korea has spurred efforts to develop an upgraded version of the missile.

The projectile was considered to have flown 500km and, if fired at a range-maximising angle, it would have travelled more than 1,000km, analysts said.

Fuelling the concern is the "unusual activity" of the North's Romeo-class submarine, which has been underway for about 10 days after sailing some 100km into waters between North Korea and Japan. It usually returns to its base within four days and rarely conducts missions in such remote waters.

"We assess that North Korea has continued its activity to develop an SLBM. We have continued to monitor relevant activities," Colonel Roh Jae Chun, spokesman for South Korea's Joint Chiefs of Staff (JCS), said at a regular press briefing.

Meanwhile, South Korea's military said on Thursday that there is no clear indication of another North Korean ballistic missile launch at hand.

Speculation had been rampant that the North would fire a missile on Thursday, the 64th anniversary of the armistice agreement that halted the 1950-1953 Korean War.

According to the military, there had been rain over the past few days at the test site in Kusong, North Pyongan Province, where the North moved a truck-mounted transporter erector launcher to prepare for a launch.

But the military highlighted that Pyongyang is ready to fire a ballistic missile "at any time and any place", and could use the mobile launchers to hide the missile test. South Korea and the US have ramped up their surveillance of the North's missile test site, it added.

"Currently, there's no sign of an imminent North Korean missile launch," Col Roh said. "Our military is continuing to closely monitor North Korea's every possible provocation, mobilising combined surveillance assets with the US."

On Wednesday, the Pentagon also dismissed the speculation that the communist country would test-launch another ballistic missile this week, saying the regime's missile and nuclear programmes are "not tied to a calendar".

"We see in North Korea a bona fide research and development programme that is not tied to a calendar," said Pentagon spokesman for navy, Captain Jeff Davis. "It's aggressively pushing ahead with test launches wherever and whenever it can. That's our concern, not the date on the calendar they pick."

The US government, meanwhile, has been trying to prevent North Korea from attending the Asean Regional Forum (ARF) early next month (August), the region's largest security conference and virtually the sole multinational event participated by the North, a Japanese newspaper reported, citing an anonymous diplomatic source.

According to Asahi Shimbun, the US has been asking the 27 ARF member states to prevent the North from joining the annual forum.

South Korean Foreign Ministry spokesman Cho June Hyuck declined to verify the report.

North Korean Foreign Minister Ri Yong Ho plans to attend the security forum in Manila, diplomatic sources said. If he attends the forum, it will be the first encounter between Mr Ri and South Korean Foreign Minister Kang Kyung Wha, who took office last month (June).