Seoul, Dec. 15 (UPI) -- SEOUL, Dec. 15 -- Top U.S. nuclear negotiator Stephen Biegun arrived in Seoul on Sunday amid rising tensions over North Korea's apparent rocket engine tests this month ahead of its year-end deadline for Washington to show flexibility in advancing their stalemated nuclear talks.
His three-day visit comes as concerns are mounting over the possibility of the North engaging in provocative acts such as a long-range rocket launch amid the limited progress in its negotiations with the U.S.
Upon arriving at an airport west of Seoul, the special representative for North Korea, now the nominee for deputy secretary of state, refused to answer questions from reporters.
National Security Council Senior Director for Asian Affairs Allison Hooker and other officials were seen accompanying Biegun.
Further aggravating tensions, the North said Saturday that it carried out "another crucial test" at its western Sohae Satellite Launching Station, better known as the Dongchang-ri site, the previous day. It claimed the test will further bolster its "reliable strategic nuclear deterrent."
North Korea said Saturday that it will use the new technologies it recently tested to develop a strategic weapon to counter the U.S. nuclear threat, escalating tensions with the U.S. ahead of its year-end deadline.
Pak Jong-chon, chief of the North Korean military's General Staff, also said North Korea has "stored up a tremendous power" and the military is fully ready to put into action any decision of leader Kim Jong-un.
"The priceless data, experience and new technologies gained in the recent tests of defense science research will be fully applied to the development of another strategic weapon of the DPRK for definitely and reliably restraining and overpowering the nuclear threat of the U.S.," Pak said in a statement carried by the North's official Korean Central News Agency.
The test followed an "important" test at the same site on Dec. 7
The North has used the Dongchang-ri site to test rocket engines and launch vehicles to deliver satellites, and such technology could be diverted to develop and launch long-range weapons.
In March 2017, North Korea successfully tested a new high-thrust engine at the station, which was employed to launch ICBMs, such as the Hwasong-14 and Hwasong-15, later that year.
Amid stalled denuclearization talks, North Korea has carried out a series of weapons tests in recent months mostly involving short-range missiles. The latest one took place on Nov. 28, when Pyongyang tested its super-large multiple rocket launcher.
Biegun's trip has spawned speculation that he could visit the inter-Korean border truce village of Panmunjom for possible contact with North Koreans. But it remains to be seen whether such contact can materialize amid no signs of either side ceding ground.
On Monday, Biegun will meet his South Korean counterpart, Lee Do-hoon, to discuss how to keep up the momentum for dialogue with the North and make progress in joint efforts to denuclearize it and foster a lasting peace on the peninsula.
The U.S. official also plans to pay a courtesy call on President Moon Jae-in on Monday, according to Moon's office.
It will be the second time Moon has met bilaterally with Biegun -- the first such meeting taking place in September last year just days before Moon's trip to Pyongyang for a summit with North Korean leader Kim Jong-un.
According to Japanese broadcaster NHK on Sunday, Biegun told reporters upon leaving Washington that the U.S. has maintained its stance regarding the denuclearization of North Korea, and the North is also aware of that.
Asked about chances of his meeting with North Korean officials at Panmunjom, he declined to give comment, it added.
Concerns have persisted that the North could pivot away from the dialogue process as it has been ratcheting up tensions with a threat to take a "new way" if the year-end deadline is not met.
The U.S. and the North last held working-level nuclear talks in Sweden in October. But the talks yielded little progress, with the North accusing the U.S. of having come to the negotiating table "empty-handed."
Since then, Pyongyang has toughened its demands, telling the U.S. to remove "all obstacles" that threaten the security of the North and hamper its development. The demands are seen as calls for sanctions relief and security assurances.
Recent weeks have seen a worrisome war of words between the U.S. and the North.
On Thursday, the North upbraided the U.S. for leading Wednesday's U.N. Security Council session on its military activities, which was widely seen as a warning that Washington could take steps such as tightening sanctions in the event that the regime conducts a long-range rocket launch.
Apparently mindful of Pyongyang's possible military provocations, U.S. President Donald Trump has revived the threat of military action, and even said that North Korean leader Kim Jong-un could lose "everything" if he engages in hostile acts.