For decades, South Koreans have lived in a technical state of war with a hostile brother country that considers them traitors and imperialist lackeys.

Throughout verbal attacks and periodic military ones, this nation of 50 million people has brushed off tensions, much as one might ignore a combative uncle at Christmas.

It's similar this time around, as North Korea launches missiles and fires off increasingly incendiary threats, and as the United States responds with stark warnings and the strategic placement of an aircraft carrier.

But now, there is one new wild card that South Koreans haven't had to factor in before: US President Donald Trump.

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"I think I've become desensitised because North Korea's nuclear threats have been going on for such a long time, and even though North Korea is a rogue state, they have to think about their survival," said Kim Jin Young, a 20-year-old political science student at Yonsei University in Seoul.

For this reason, she doesn't worry much about what Kim Jong Un might do.

"But Trump is capable of a lot of things, and his words have a huge impact on the economy and defence policy of South Korea," she said while studying in the campus cafeteria on a rainy Monday. "I hope he doesn't make decisions by himself and that he doesn't abuse his power."

In the three months he has been President, Trump has proved himself quick to pull the trigger, if his early-morning tweets and his surprise airstrikes on Syria and Afghanistan are any guide.

Vice-President Mike Pence said in Seoul yesterday that North Korea should take note of Trump's decisiveness in those cases, and warned Pyongyang "not to test his resolve".

Kim Min Seob, an IT worker stopping for a coffee break with his colleague, said this concerned him. "Both Trump and Kim Jong Un are escalating the tensions by speaking about a possible war," he said.

"So the situation has become more serious under Trump. We know he does not stop at talking," said Kim, 53. "He showed that by bombing Syria for using chemical weapons."

The Internet has delighted in comparing Kim and Trump, as off-base as that might be. But both leaders have a decidedly blunt way of putting things.

"Trump speaks very aggressively, but I think it's because of his lack of diplomatic experience," said Song Baek Beom, a 26-year-old senior at Yonsei University. "He talked loudly during the campaign about prosecuting Hillary [Clinton] but he chose not to press charges against her. I don't think Trump will turn all of his aggressive remarks into actions."

The fact that South Korea hasn't even had a president since Trump took office - the previous one, Park Geun Hye, was busy getting impeached, and the next one won't be elected until May 9 - hasn't helped.

Although the leaders of Japan and China have been to Trump's Mar-a-Lago resort in Palm Beach, Florida, South Korea's acting president has had only a few phone calls with Trump.

South Koreans noticed when the president didn't even mention them during a hasty news conference with Japanese Prime Minister Shinzo Abe on the night in February that North Korea launched the first ballistic missile of his Administration.

But for every South Korean who's worried about the uncertainty that Trump injects into this equation, there's another who knows the American President is surrounded by generals to whom he actually listens.

"I heard Trump is a hard-liner but would he take actions that would put South Korea in danger?" asked Kim Yoo Hwan, 60, who runs a printing store. "If the US attacks North Korea, the North will hit South Korea in return, and then we're in a war situation. So I don't think Trump would start a war that easily."

And what about Kim, who has shown himself to be both hotheaded and ruthless? South Koreans are not so sure what the 33-year-old might do.

"I worry more about Kim Jong Un than Trump, as it seems he's willing to risk a war if attacked," said Lee Ji Hoon, the manager of a dog cafe and hotel.

"The recent assassination of Kim Jong Nam shows what Kim Jong Un is capable of," Lee said. "I really hope it doesn't happen, but I'm worried that Kim Jong Un might start a war if he comes under an attack."

But despite the new questions Trump has introduced, it's business as usual in Seoul right now.

The cars are caught in their regular traffic jams, the sidewalks are full of people staring into their cellphones, and the barbecue restaurants suffer their normal 6.30pm rush of office workers.

"We're safe here!" Lee Ok Soo, who runs a dry-cleaning store with her husband, said, and laughed when asked whether she is concerned about the tension on the Korean Peninsula.

"I support what Trump is doing now," the 58-year-old said. "I hope he ties up the hands of North Korea so they can't develop nuclear weapons anymore."