Acting US Defense Secretary Patrick Shanahan said Wednesday that North Korea's missile launches this month were a violation of UN Security Council resolutions, adding to the confusion over the US views.

North Korea launched short-range missiles on May 9 in apparent protest over stalled negotiations with the US While US National Security Adviser John Bolton condemned them as a breach of UNSC resolutions, which ban the North's use of ballistic missile technology, President Donald Trump said he views them "differently" and referred to the missiles as "small weapons."

Asked to clarify, the State Department said Tuesday that the entire North Korean weapons of mass destruction program is in conflict with the resolutions but stopped short of singling out the missile launches.

"Let me just be clear, the short -- these were short-range missiles and those are a violation of the UNSCR. Yes," Shanahan told reporters en route to Jakarta, Indonesia.

At a press briefing later in the day, State Department spokeswoman Morgan Ortagus acknowledged Shanahan's remarks but reiterated her comments on the North Korean WMD program being in violation of the resolutions.

"We remain focused on diplomacy," she said, adding it was "important to point out ... that we do see a bright future for the North Korean people" after the regime gives up its nuclear program and gets sanctions relief.

Trump has appeared eager to keep diplomacy with the North alive as he touts the regime's suspension of nuclear and long-range missile tests since 2017 as one of his major accomplishments.

Negotiations between Washington and Pyongyang hit an impasse after a second Trump-Kim summit in February collapsed due to differences over the scope of North Korea's denuclearization and sanctions relief from the US.

Shanahan is due to travel to South Korea next week as part of an Asia tour. (Yonhap)