Washington (CNN) A group of Democratic US senators on Thursday sent a letter to President Donald Trump urging him to escalate diplomatic action against North Korea following months of missile launches from the country, which have received little pushback from the administration.

"We write to express our grave concern regarding your policy and diplomacy with North Korea," the senators write, "and to urge you to redouble efforts to forge a successful and durable path towards denuclearization of North Korea -- by diplomatic means -- while the opportunity still exists."

"Your administration has downplayed the significance of these tests and suggested that there is no rush to reach an agreement that verifiably freezes and reverses North Korea's nuclear and missile development," the letter says. "Accepting North Korean ballistic missile tests represents, in our view, a significant step backwards in the negotiations, especially as you yourself have previously asserted that North Korea halting all ballistic missile tests and nuclear tests was a sign of your administration's success."

The senators write that Trump's ability to address North Korea's missile testing "is vital for our national security interests, and we want to support your administration in the execution of a coherent, durable and sustainable strategy."

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