SEOUL/TOKYO (REUTERS) - China and South Korea agreed on Monday (April 10) to slap tougher sanctions on North Korea if it carries out nuclear or long-range missile tests, a senior official in Seoul said, as a United States Navy strike group headed to the region in a show of force.

North Korea marks several major anniversaries this month and often marks the occasions with major tests of military hardware.

The possibility of US military action against North Korea in response to such tests gained traction following last week's strikes against Syria. Previously, Washington has leaned toward sanctions and pressure to deter North Korea, but comments from US President Donald Trump's top aides at the weekend suggest that position may be hardening.

However, South Korea's chief nuclear envoy Kim Hong Kyun said there was no mention of any military option in his talks with China's Special Representative for Korean Peninsula Affairs, Wu Dawei.

The two also did not discuss any possible strike against the North by the Trump administration, he said.

"Both sides agreed that despite the international community's warnings, if North Korea makes strategic provocations such as a nuclear test or an ICBM launch, there should be strong additional measures in accordance with UN security council resolutions," Kim told reporters.

Kim added the two sides agreed "an even stronger UN resolution" will have to be adopted in the event of additional weapons test by North Korea.

Wu did not speak to reporters.

US Secretary of State Rex Tillerson has said the US military strike against Syria over its alleged use of chemical weapons was a warning to other countries including North Korea that "a response is likely" if they pose a danger.

"(Chinese) President Xi (Jinping) clearly understands, and I think agrees, that the situation has intensified and has reached a certain level of threat that action has to be taken,"Tillerson said on CBS's Face the Nation.

The US Navy strike group Carl Vinson cancelled a planned trip to Australia and was moving toward the western Pacific Ocean near the Korean peninsula as a show of force, a US official told Reuters over the weekend. "We feel the increased presence is necessary," the official said.

Trump and China's Xi held a summit meeting in Florida last week, where Trump pressed his counterpart to do more to curb North Korea's nuclear programme.

China is North Korea's main diplomatic and economic ally.

OPPOSITION TO THAAD

Wu's trip was the first visit to South Korea by a senior Chinese official since the planned deployment of the US Thaad missile defence system led to a diplomatic row between Beijing and Seoul.

Kim said Wu repeated China's position on the Thaad deployment but did not give details. China has previously said the system would destabilise the regional security balance and that its radar's reach would be intruding into Chinese territory.

North Korea has sounded a note of defiance against the United States, calling the strikes against Syria on Friday "an unforgivable act of aggression" that showed Pyongyang's decision to develop nuclear weapons was "the right choice."

In Tokyo, the feasibility of US military action was downplayed, while South Korea said the focus remained on deterrence and readiness. "It probably is not realistic for the US to attack North Korea," a Japanese defence ministry source said.

"If America says it is going to attack, both Japan and South Korea will probably put a stop to it," said the source, who declined to be identified.

A senior Japanese military source added: "If the US military was to attack, there could be a request to Japan for rear-guard logistics support but there has been no talk of such preparations."

South Korean and US forces are also involved in annual joint military drills that run until the end of April. The North calls the drills preparations for war against it.

Several North Korean anniversaries in April could be an opportunity for Pyongyang to conduct nuclear or missile tests, South Korean defence ministry spokesman Moon Sang Kyun said.

North Korea has invited a large number of foreign media representatives to Pyongyang this week, likely to cover the so-called "Day of the Sun" birth anniversary on April 15 of the state founder Kim Il Sung.

North Korea launched a ballistic missile on Wednesday that flew a short distance before spinning out of control and crashing into the sea, the latest of a number of missile tests defying UN sanctions.

The North is also seen to be ready to conduct its sixth nuclear test at any time, with movements detected by satellites at its Punggye-ri nuclear test site.