By Phil Stewart and Idrees Ali

WASHINGTON, March 16 (Reuters) - North Korea might begin flight testing an improved design for its inter-continental ballistic missiles "in the near future," the head of the U.S. military's Northern Command said on Tuesday, a move that would sharply increase tensions between Pyongyang and Washington.

The warning by Air Force General Glen VanHerck appeared based on North Korea's October unveiling at a parade of what would be its largest ICBM yet, and not specific intelligence about an imminent launch.

Still, his remarks came a day ahead of a debut trip by President Joe Biden's top diplomat and defense secretary to South Korea and underscore U.S. concerns that Pyongyang may resume testing of missiles and nuclear weapons after a hiatus of more than three years.

Even during the testing lull, North Korean leader Kim Jong Un has called for continued production of nuclear weapons for his arsenal, launched a series of smaller missiles and unveiled what would be North Korea’s largest ICBM yet at the October parade.

VanHerck told the Senate Armed Services Committee that Pyongyang's "considerably larger and presumably more capable" ICBM further increased the threat to the United States. Still, he expressed confidence in U.S. missile defenses.

The U.S. general also noted that Kim released himself from a moratorium on testing over a year ago.

"The North Korean regime has also indicated that it is no longer bound by the unilateral nuclear and ICBM testing moratorium announced in 2018, suggesting that Kim Jong Un may begin flight testing an improved ICBM design in the near future," VanHerck said in his written testimony.

The White House confirmed a Reuters report on Monday that it has sought to reach out to North Korea but had received no response, extending a chill in relations that began at the end of Donald Trump's administration.

After the White House remarks, North Korean state news reported that the sister of the North Korean leader, Kim Yo Jong, criticized the Biden Administration for ongoing military drills in South Korea.

"If it wants to sleep in peace for coming four years, it had better refrain from causing a stink at its first step," Kim said in a statement carried by state news agency KCNA.

The joint U.S.-South Korean springtime military drill begun last week was limited to computer simulations because of the coronavirus risk, as well as the ongoing efforts to engage with the North.

(Reporting by Phil Stewart and Idrees Ali; Editing by Dan Grebler)