In the wake of the latest North Korean missile launch, Sens. Jon Tester and Heidi Heitkamp are urging President Donald Trump to appoint a special envoy to the Korean peninsula to continue diplomatic efforts as tensions grow.

In a letter to the president, the two Democratic senators from Montana and North Dakota said a report from the Defense Intelligence Agency suggests that North Korea could develop a ballistic missile capable of striking the U.S. mainland by next year.

Tester is calling on the Trump administration to appoint a special envoy to the region “tasked with easing regional tensions while utilizing the full toolkit of the U.S. government to resolve the current crisis.”

“We write with deep concern about North Korea’s dangerous behavior and acceleration of its nuclear weapon and ballistic missile programs, which threaten our national security,” the senators wrote. “The United States cannot sit idly by as a dangerous dictator threatens our families, our freedoms, and America’s standing in the world.”

On Friday, North Korea tested another long-range missile that reached a higher altitude than a previous test conducted in early July.

By Sunday, U.S. defense officials said they had conducted a successful test of a separate missile capable of tracking and intercepting any hostile rocket launched by North Korea.

In their letter, Tester and Heitkamp said they don't believe the Trump administration has a clear strategy to address North Korea, its development of rocket technology and its nuclear ambitions.

Critical positions remain unfilled, they said, including the ambassador to South Korea, a special envoy to North Korea, and the special envoy for six party talks. The talks were established in 2003 after North Korea pulled out of the Nuclear Non-Proliferation Treaty.

“It's important for your administration to prioritize a regional strategy by appointing a special envoy to the Korean Peninsula in order to ensure the safety and security of American lives,” they said. “We agree that 'strategic patience' is not a suitable course of action and instead we should exhaust every diplomatic tool our county has at its disposal.”