The United States is concerned by North Korea's missile development and will continue to call out the regime for its missile tests, a senior US official said Tuesday.

Andrea Thompson, under secretary of state for arms control and international security, was commenting in light of North Korea's five rounds of short-range ballistic missile tests since July 25.

Experts say the regime has been advancing its weapons capabilities despite a commitment by North Korean leader Kim Jong-un to work toward denuclearization.

The tests were also seen as an attempt to strengthen Pyongyang's position ahead of a potential resumption of denuclearization talks with Washington.

"I won't go into the intelligence work in this forum, but what I can tell you is we are concerned about the missile development," Thompson told reporters in a teleconference. "We'll continue to call out the DPRK on their missile testing. We continue to work, again, with partners and allies as we uphold the UN Security Council sanctions, working hand in hand to ensure that that pressure campaign remains firm with North Korea."

DPRK stands for North Korea's official name, the Democratic People's Republic of Korea.

Still, Thompson said, the US will continue engaging the regime in talks and hope the North Korean leader delivers on his commitment to denuclearize.

"We want the talks to happen," she said. "We have working-level discussions and have faith and confidence that the president and Chairman Kim will engage again."

US President Donald Trump and Kim agreed to restart working-level negotiations within several weeks when they held their third meeting at the inter-Korean border on June 30.

With the talks yet to be scheduled, Trump revealed on Twitter last weekend that in a recent letter to him, Kim expressed his desire to resume the talks as soon as the South Korea-US military exercises end later this month.

Thompson said she didn't have a date for the talks' resumption but noted she felt encouraged by the relationship between Trump and Kim.

"So we'll continue to pursue that. Negotiations will continue to move forward, and we will continue to hold Chairman Kim accountable for the promises that he made to our president," she said.

"I have great faith and confidence in our Special Envoy Steve Biegun and the technical experts behind him," she added.

US Special Representative for North Korea Stephen Biegun is in charge of day-to-day negotiations with Pyongyang. Recent media reports have suggested he could be picked by Trump to be the next US ambassador to Russia. (Yonhap)