The Yonhap news agency in South Korea reported that Monday marked the third consecutive day that North Korea launched short-range missiles over the Sea of Japan.
A North Korean underground test of a nuclear device in February sparked concerns about the security situation in the region. Tensions escalated further during joint U.S.-South Korean drills, though the situation had calmed down in recent weeks.
A South Korean defense official told Yonhap on condition of anonymity that the launch was seen as an effort to "renew military tensions on the Korean Peninsula."
U.S. State Department spokesman Patrick Ventrell said North Korea was called on to urge restraint. The overall security situation was unchanged, however.
"I'm not aware that we've given any new update to American citizens in South Korea or anything of that nature," he said.
Ventrell added that Robert King, a U.S. special envoy on North Korean human rights, had to cancel a visit to Seoul and Tokyo because of a "logistical matter."
North Korea is suspected of supporting its military arsenal to the detriment of the welfare of its people.