While State Department spokeswoman Marie Harf refused to speculate on the reasons behind the purging of Jang Song-Thaek, she acknowledged that "stability on the Korean peninsula is very important to us."
"We're going to increase our discussions with our allies and partners in the region about the internal situation in North Korea," she told reporters.
Washington is in regular talks with Beijing, a close ally of the isolated Pyongyang regime, and "we're on the same page in terms of urging the North Koreans to come back in line with their international obligations," Harf said.
Jang -- who had been seen as Kim's political regent and the country's unofficial number two -- was executed on Thursday immediately after a special military trial, state news agency KCNA reported.
Jang, 67, played a key role in cementing the leadership of the inexperienced Kim when he succeeded his father Kim Jong-Il in 2011, but analysts said his power and influence had become increasingly resented.
Harf denounced the execution as an "incredibly brutal act" and said it "underscores the horrific human rights record of the North Korean regime."
"What we really see is that North Korea has a choice between continuing down a path of isolation and impoverishment for their own people, or meeting its obligations and coming back into the international system," she added.
Harf said China was stressing that point directly with North Korea.
