Spiking nuclear tensions between the United States and North Korea showed little sign of abating Sunday as Russia warned of a "very unpredictable nosedive" if Washington does not ease up on its fiery war of words with Pyongyang.

US bombers and fighter escorts flew off the coast of North Korea on Saturday and US President Donald Trump took to Twitter with verbal threats as the brinkmanship with Pyongyang intensified.

"Just heard Foreign Minister of North Korea speak at UN. If he echoes thoughts of Little Rocket Man, they won't be around much longer!", Trump wrote.

The bellicose rhetoric comes as international alarm mounts over Pyongyang's weapons ambitions - including a suggestion last week that the country is considering detonating an H-bomb over the Pacific.

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Russian Foreign Minister Sergei Lavrov cautioned that a softer touch was needed to defuse the crisis.

Only "caresses, suggestion and persuasion" will work, Lavrov told Russia's NTV television in an interview that aired Sunday.

If the US does not ease up, he said, "we could drop into a very unpredictable nosedive and tens if not hundreds of thousands of innocent citizens of South Korea but also North Korea, of course, and Japan will suffer - and Russia and China are nearby."

Despite the heated exchanges, Lavrov said the United States would not take military action against North Korea because "they know for sure that it has nuclear bombs."

"I'm not defending North Korea. I'm just saying that almost everyone agrees with such an analysis," he said.

Show of force

In a show of force, US B-1 bombers took off from the Pacific territory of Guam and flew over international waters off the east coast of North Korea, accompanied by F-15C fighters based in Okinawa, Japan.

US bombers have carried out similar flights before, but the Pentagon stressed this was the furthest north of the Demilitarized Zone (DMZ) between the two Koreas that any US fighter or bomber has flown off North Korea's coast in this century.

"This mission is a demonstration of US resolve and a clear message that the president has many military options to defeat any threat," Pentagon spokeswoman Dana White said.

"We are prepared to use the full range of military capabilities to defend the US homeland and our allies."

Washington announced tougher restrictions Friday aimed at curbing North Korea's nuclear and ballistic missile program, building on tough new UN sanctions aimed at choking Pyongyang of cash.

US Treasury Secretary Steven Mnuchin said the executive order "gives the Treasury more authorities than we've ever had before."

"The president is very committed to blocking economic transactions and that's what this is all about," he said on CNN's State of the Union show.