U.S. officials are both alarmed and surprised by North Korea’s recent military activity, with a high-ranking U.S. general saying that Pyongyang is rapidly expanding its weapons capabilities.

Air Force Gen. John Hyten, vice-chairman of the Joint Chiefs of Staff, said Friday at the Washington-based Center for Strategic and International Studies that North Korea “has been building new missiles, new capabilities, new weapons as fast as anybody on the planet with the 115th most powerful economy in the world. Speed itself is efficiency.”

Other U.S. officials have been surprised that North Korea has not flexed its military capabilities during the holiday season. North Korea had promised the United States a “Christmas gift” in early December, which military analysts believed was an intercontinental ballistic missile (ICBM) launch. Pyongyang did not end up delivering on its promise.

“You can say that I personally was surprised. But I’m glad also ... there was no Christmas gift,” U.S. Ambassador to South Korea Harry Harris said Thursday. “Washington was ready for any eventuality, and we were all glad that there was no ICBM test or nuclear test.”

Pyongyang has grown angry over the sanctions that have been levied on its economy. The U.S. and North Korea have been in negotiations to work out a nuclear deal, with some sanctions from the North Korean economy being removed in exchange for Pyongyang pursuing a path to denuclearization.

Nuclear negotiations have been stalled since early October, when U.S. and North Korean negotiators met in Sweden to discuss a deal, but talks fell through on the first day. President Trump and North Korean leader Kim Jong Un have also met in Vietnam and Singapore for discussions without reaching a deal.

North Korea has recently said that it felt “deceived” by the U.S. regarding the lack of progress towards relieving sanctions. Trump has sent Kim a “birthday letter” to break the ice, but this has failed to open the door for future talks.

Former National Security Adviser John Bolton has said that North Korea will never give up its nuclear weapons arsenal, and that the Trump administration is not doing all it can to confront Pyongyang.