United States Secretary of Defense Mark Esper leaves the podium after delivering a speech during a ceremony to commemorate the 75th anniversary of the Battle of the Bulge at the Luxembourg American Cemetery in Hamm, Luxembourg, Monday, Dec. 16, 2019. AP

U.S. Defense Secretary Mark Esper said on Monday that North Korea's rhetoric was concerning and Pyongyang would likely carry out tests if they "don't feel satisfied," amid fears the two countries could return to the collision course they had been on before launching diplomacy.

"We have seen talk of tests. I think that they will be likely if they don't feel satisfied," Esper told reporters traveling with him from Luxembourg back to Washington. He did not provide details on what type of tests may be likely.

Trump says US is watching North Korea closely 2019-12-17 09:25  |  North Korea
"I've been watching the Korean Peninsula for maybe a quarter of a century now. So I'm familiar with their tactics, with their bluster and I think we need to get serious and sit down and have discussions about a political agreement that denuclearizes the Peninsula," Esper said. (Reuters)