U.S. Special Representative for the Democratic People's Republic of Korea (DPRK) Stephen Biegun said on Wednesday he will not take up the post of ambassador to Russia but will remain focused on making progress on the denuclearization of the DPRK amid stalled negotiations.

Biegun, in Seoul for talks with officials of the Republic of Korea (ROK), said the United States is prepared to restart talks with the DPRK as soon as the DPRK is ready, in line with an agreement made by the two countries' leaders in June.

"I will not be taking up a diplomatic posting in the Russian Federation and I will remain focused on making progress on North Korea (the DPRK)," Biegun told reporters after meeting with his ROK counterpart Lee Do-hoon.

Biegun, with his expertise in Russian affairs and command in the language, was under consideration to be U.S. President Donald Trump's next ambassador to Russia, a source familiar with the matter said last week.

Fox News reported on Tuesday, citing an administration official, that Trump will nominate Deputy Secretary of State John Sullivan for that post, currently held by Jon Huntsman, who has resigned amid speculation he plans to run for Utah governor.

Biegun leads working-level talks with the DPRK which have stalled since a failed second summit between Trump and DPRK leader Kim Jong Un in Hanoi in February.

The two leaders met again in June at the inter-Korean border and agreed to reopen negotiations, but this has yet to happen.

"I am fully committed to this important mission, and we will get this done," Biegun said. "We are prepared to engage as soon as we hear from our counterparts in North Korea."

Trump said early this month that Kim had told him he was ready to resume talks and would stop ongoing missile tests as soon as U.S.-ROK military drills end.

On Tuesday the allies wrapped up their largely computer-simulated, 10-day exercises, with which they replaced previous large-scale annual drills to expedite the nuclear talks.

Lee said Biegun's visit was made at a "very timely manner," and both sides discussed how to swiftly restart dialogue with the DPRK and make "substantive progress."

Pyongyang has condemned the joint exercises as rehearsal for invasion. On Wednesday, its official Rodong Sinmun said in a commentary that the latest drills were a "grave military provocation," threatening to "develop, test and deploy" self-defensive measures.

Biegun arrived in Seoul after a stop in Japan, and is scheduled to travel to China on Thursday.

He said he "appreciated the strong and continued" trilateral cooperation involving Japan amid historical and trade tension between Seoul and Tokyo.