South Korea will order billions of dollars of US military equipment to improve defense capabilities against North Korea, US President Donald Trump announced in Seoul on Tuesday.

At a joint news conference with South Korean President Moon Jae-in, Trump warned North Korea that he was prepared to use the "full range" of military might to defend the United States and its allies.

"I think we're showing great strength. We sent three of the largest aircraft carriers in the world ... and we have a nuclear submarine positioned," Trump told reporters. "We hope to God we never have to use them."

"With that being said, I really believe that it makes sense for North Korea to come to the table and to make a deal that is good for the people of North Korea and the people of the world," he said.

Read more: Trump's 24 hours in South Korea - North Korea, trade high on agenda

China and Russia

Trump called on regional powers to help deal with North Korea's aggression. "We call on every responsible nation including China and Russia to demand that the North Korean regime end its nuclear weapons and missile programs and live in peace," he said. "All nations must implement the United Nations Security Council regulations and cease trade and business entirely with North Korea. It is unacceptable that nations would work to arm and finance this increasingly dangerous regime."

Later, Kremlin spokesman Dmitry Peskov said Russian President Vladmir Putin and Trump would likely discuss North Korea at the Asia-Pacific Economic Cooperation forum in Vietnam this week.

Trump and Moon spoke optimistically of dialogue with North Korea backed up by military might.

"Today we had candid discussions about the steadfastness of the US alliance," Moon said. "We worked towards resolving the North Korea nuclear issues in a peaceful manner."

"We reaffirmed our principle that we must maintain a strong stance towards North Korea based on overwhelming strength," Moon said.

Read more: US, Japan agree that 'all options' are on the table for North Korea

Increase military spending

In addition to increasing South Korea's military spending, the two also agreed to reduce the limit on missile payloads, maximize pressure on North Korea, and step up US troop deployments. They also plan to expedite trade negotiations, with Trump lambasting the current deal for being detrimental to the US.

"We make the greatest military equipment in the world, whether it's planes, missiles. South Korea will be ordering billions of dollars of that equipment, which frankly makes sense for them, and for US jobs and reducing the trade deficit," Trump said.

Anti-Trump protests in Seoul

The US president flew in directly from Japan where he secured Tokyo's full support for Washington's stance that "all options are on the table" regarding Pyongyang, and declaring its nuclear ambitions "a threat to the civilized world and international peace and stability."

In Seoul more than 15,000 police were on high alert as both critics and supporters of Trump came out to protest.

Several dozen anti-Trump protesters rallied near the presidential office, holding signs that read "Trump, NOT welcome!" and "Say no to Trump, say no to war." They accused him of stoking tensions with North Korea and pressuring Seoul to buy more US weapons. On trade, they accused him of pressing Seoul to renegotiate a bilateral free trade deal to favor the US.

Trump supporters, largely conservative older South Koreans, were also planning rallies in nearby streets.

It 'has to work out'

Trump visited the US military base Camp Humphreys, about 70 kilometers (43 miles) south, earlier in the day.

"Ultimately it will all work out. Because it always works out - has to work out," he said without elaborating.

Ahead of the meeting Trump promised on Twitter to figure out all the issues.

aw/kms (Reuters, AP, dpa, AFP)