SEOUL, South Korea -- President Donald Trump said North Korea has been making "fools of U.S. negotiators" and "only one thing will work," the latest hint of possible military action against the communist state. The comments came amid fears that the North may test-fire another missile in connection with the anniversary on Tuesday of the foundation of its ruling party. "Presidents and their administrations have been talking to North Korea for 25 years, agreements made and massive amounts of money paid," Trump wrote in a tweet posted Saturday. "It hasn't worked, agreements violated before the ink was dry, [making] fools of U.S. negotiators. Sorry, but only one thing will work!" he added. It was the latest signal that the president is taking a hard line on dealing with the North, despite efforts by his secretary of state and other advisers to find a diplomatic solution to the crisis over the country's nuclear weapons program. Trump also told reporters at a gathering last week with military leaders that this "could be the calm before the storm." He declined to clarify his remarks but many speculated that he was referring to North Korea. A Russian lawmaker said the North is preparing to launch a long-range missile "in the nearest future," based on his recent visit to Pyongyang. Anton Morozov of Russia's nationalist Liberal-Democratic Party said he and his colleagues also were shown North Korean calculations indicating they have missiles that could reach the U.S. West Coast, the Russian state news agency RIA Novosti reported Friday. Morozov said the delegation was told the North has the technology that would allow its missile warheads to survive intense heat when re-entering the atmosphere. Many experts doubt the North has perfected that essential capability but say it has made major advances in its nuclear weapons program, despite several rounds of tightened economic sanctions and other punishing measures. South Korea also has said it is closely monitoring the possibility of another provocation, as the North gears up to celebrate the ruling Workers' Party foundation day. But the president's office said there has been no clear sign of activity. Secretary of State Rex Tillerson has pushed for a diplomatic solution that would rely on punishing sanctions and international pressure to curb the North's ambitions. But Trump tweeted last week that Tillerson was "wasting his time trying to negotiate with Little Rocket Man," using the president's derogatory nickname for North Korean leader Kim Jong Un. Many North Korea watchers have expressed concern about Trump's decision to use cryptic and threatening language, saying it raises the risk for a miscalculation that could lead to open conflict. North Korea, meanwhile, warned that Trump's statements amounted to "extremely dangerous military gambling." "The U.S. strategic assets that were deployed on the Korean peninsula will be the first target of destruction by the revolutionary armed forces of [North Korea] which has the U.S. mainland across the Pacific within their striking range and the puppet forces will face total destruction," the state-run Korean Central News Agency reported, citing a statement by an unnamed spokesman for the Pan-Korean Emergency Measure Committee for Opposing Nuclear War Drills against the North. Past efforts at negotiating an end to North Korea's missile and nuclear development have had limited success in exchange for concessions from the West. Washington and Pyongyang signed a deal known as the "agreed framework" in 1994, in which the North committed to freezing its plutonium weapons program in exchange for aid. But that agreement collapsed in 2002. Subsequent six-party talks involving the two Koreas, China, Russia, the United States and Japan fell apart in 2009 after North Korea launched a rocket shortly after President Barack Obama's inauguration.