A day after North Korea fired a missile over Japan, US President Donald Trump on Wednesday said that "talking is not the answer" in dealing with Pyongyang's military operations.
"The US has been talking to North Korea, and paying them extortion money, for 25 years," he wrote on Twitter.
But US Defence Secretary James Mattis, insisted on Wednesday that there was still room for diplomacy.
North Korea has said its firing of a missile over Japan on Tuesday was "the first step" of military operations in the Pacific.
Russian Foreign Minister Sergei Lavrov has told US Secretary of State Rex Tillerson over phone that any further strengthening of sanctions against North Korea would be counter-productive.
Trump's comments come just days after he said that North Korean leader Kim Jong-un was "starting to respect" the US.
On Tuesday, Pyongyang fired a ballistic missile over Japan's northern Hokkaido island, triggering public alerts to take cover before landing in the sea.
Speaking ahead of a meeting at the Pentagon with his South Korean counterpart, Song Young-moo, Mattis said, "We are never out of diplomatic solutions."
Russia has said that US military activity in the region is partly to blame for the increase in tensions, and has urged restraint and called for negotiations.
Lavrov stressed on Wednesday that diplomacy was the only way to overcome tensions on the Korean peninsula, which have been escalating in recent months.
North Korean state media this week also repeated threats to the US Pacific island of Guam, which it called "an advanced base of invasion".
