SEOUL - A senior U.S. diplomat Monday made a last-minute plea for North Korea to stop its provocations and re-enter nuclear negotiations, just ahead of Pyongyang's end-of-year deadline for nuclear talks.

"Let me speak directly to our counterparts in North Korea: It is time for us to do our jobs. Let's get this done. We are here, and you know how to reach us," said Steve Biegun, the U.S. special representative for North Korea.

Biegun was speaking in Seoul, days after North Korea conducted two tests, apparently of engines for long-range missiles. Pyongyang has promised to deliver an ominous "Christmas gift" if the U.S. does not offer more concessions by the end of the year.

The moves have intensified fears that major tensions could return to Korea in 2020, possibly derailing nearly two years of diplomacy between U.S. President Donald Trump and North Korean leader Kim Jong Un.

"But it does not have to be this way. It is not yet too late," said Biegun, who was meeting with senior South Korean officials Monday.

"Let me be absolutely clear: the United States does not have a deadline. We have a goal," Biegun said.

In recent weeks, North Korea has expressed frustration at the U.S. refusal to acknowledge its end-of-year ultimatum and has increased threats as the deadline approaches.

Last week, North Korea's foreign ministry said Pyongyang had "decisively" made a "definite decision" on what to do, but didn't say what the decision was.

North Korea has conducted 13 rounds of short-or medium-range missile tests since May. It has warned it could soon resume long-range missile or nuclear tests.

Nuclear talks broke down in February when Trump walked away from a summit in Hanoi with Kim.

The two sides resumed working-level discussions in October in Stockholm, but North Korea walked away this time, complaining the U.S. had not made an appropriate offer. It has since boycotted the talks.

North Korea may give a hint about its future direction during an important meeting of the ruling Workers' Party of Korea later this month. Kim New Year's speech will also be closely watched for comments on the nuclear talks.