The United States and South Korea will maintain the "strongest possible joint deterrent" over North Korea's "escalatory actions," the U.S. envoy on North Korea said Monday, amid concerns that Pyongyang is preparing to resume nuclear testing.

U.S. Special Representative Sung Kim and his deputy, Jung Pak, met South Korean officials, including nuclear envoy Noh Kyu-duk, after arriving in Seoul early Monday for a five-day visit.

"It is extremely important for the United Nations Security Council to send a clear signal to the DPRK that we will not accept its escalatory tests as normal," Kim told reporters after his talks with Noh, referring to North Korea by its official name, the Democratic People's Republic of Korea.