Renewing his rhetoric on North Korea after a few days, United States President Donald Trump on Saturday said there was “only one thing” that could work to tame Pyongyang. However, he did not elaborate on what the “one thing” could be.

In a pair of tweets, Trump said that previous administrations had been “talking to North Korea for 25 years”, made agreements and paid “massive amounts of money”, but nothing worked. Agreements got violated “before the ink could dry”, and this made fools of the US negotiators, he said.

“Sorry, but only one thing will work!” Trump said in one of the tweets.

This comes a week after he said that negotiating with North Korea about its nuclear programme would be a waste of time. He had also appeared to undercut his Secretary of State Rex Tillerson, by declaring on Twitter that any talks – as Tillerson had suggested – would be futile. “Save your energy Rex, we’ll do what has to be done!” Trump had tweeted.

North Korean leader Kim Jong-un and Trump have been exchanging threats in recent weeks, after tensions over the Korean peninsula escalated as the North has tested several missiles this year.

North Korean state media had on September 3 claimed that they had successfully conducted a hydrogen bomb test. This was the country’s sixth underground nuclear test since 2006.

On September 19, in his first address to the United Nations General Assembly, Trump had called Kim a “rocket man” on a “suicide mission,” prompting Kim to warn the US president that he would “pay dearly” for his comments.