US President Barack Obama said America stood "shoulder to shoulder" with South Korea over North Korean provocation, as he continued his Asia tour.
Mr Obama, who arrived from Japan, spoke after holding talks with South Korean leader Park Geun-hye.
His visit comes amid concerns North Korea could be planning to conduct a fourth nuclear test, following reports of activity at its nuclear test site.
Mr Obama also expressed his condolences over last week's ferry disaster.
More than 300 people were killed or remain missing after the Sewol passenger ferry sank off South Korea, in a tragedy that has shocked the nation. Most of those who died were teenagers on a school trip.
"I am a father of two daughters close to the same age of those who were lost. I can only imagine what those parents are going through," he said.
He brought with him a magnolia tree from White House, which he said he was presenting to the students' high school to commemorate "the beautiful lives lost".
As well as talks with Ms Park, Mr Obama will visit US troops and then fly to Malaysia on Saturday.
Before he touched down in Seoul, he called the North Korean problem "the most destabilising, dangerous situation in all of the Asia-Pacific region".
Earlier this week, South Korea's military said it had detected "a lot of activity" at the North's nuclear test site, suggesting it was either planning a test or would pretend to stage one.
A report from 38 North, the website of the US-Korea Institute at Johns Hopkins University, also confirmed increased activity at the site "probably related to preparations for a detonation", based on satellite imagery.
North Korea has carried out three such tests in the past, most recently in February 2013.
All resulted in the imposition of sanctions by the UN, which bars Pyongyang from nuclear tests under resolution 1718.
Speaking in Seoul, Mr Obama said provocations by Pyongyang would only lead to further isolation.
"The US and South Korea stand shoulder to shoulder, both in face of Pyongyang's provocations and our refusal to accept a nuclear North Korea."
He said both he was encouraged that Beijing - believed to have the most influence on Pyongyang - was beginning to recognise that North Korea was "not just a nuisance but a significant problem to their own security".
And he said that in in light of "what we expect to be further provocations" in the form of missile or nuclear tests, he and Ms Park had agreed to look for additional ways to apply pressure on Pyongyang.
For her part, Ms Park said during their joint press conference: "When North Korea is threatening further provocations and publicly discussing the possibility of a further nuclear test, President Obama's visit to South Korea will send a firm message that North Korea's provocations will not be tolerated."
While Pyongyang has tested devices, it is not yet believed to have mastered the process of making a nuclear warhead small enough to deliver via a missile.