U.S. President Joe Biden walks on the South Lawn of the White House before boarding Marine One in Washington, Jan. 28. UPI-Yonhap

The United States is concerned North Korea's escalating missile tests could be precursors to resumed tests of nuclear weapons and intercontinental ballistic missiles, a senior U.S. official said Sunday (local time), while urging Pyongyang to join direct talks with no preconditions.

North Korea conducted a test of one of its largest missiles Sunday, sending a suspected intermediate-range ballistic missile (IRBM) soaring into space for the first time since 2017.

South Korean President Moon Jae-in said the recent flurry of North Korean missile tests was reminiscent of heightened tensions in that year, when North Korea conducted multiple nuclear tests and launched its largest missiles.

He said the latest launch took North Korea a step closer to fully scrapping a self-imposed moratorium on testing its longest-range intercontinental ballistic missiles (ICBMs), which it hasn't tested since 2017.

A senior official of the Biden administration was asked in a briefing for journalists whether Washington shared the concern that Pyongyang might resume ICBM and nuclear weapons testing.

"Of course, we're concerned," he said. "It's not just what they did yesterday, it's the fact that this is coming on the heels of quite a significant number of tests in this month. And that follows on tests at the end of the year going back to September, of a variety of systems."