SEOUL (Reuters) - South Korea said on Friday it (Oct 27) will accept the release of a South Korean fishing boat captured by North Korea later in the day, with a government spokesman saying it is "a relief" the crewmen on board would be returned.

The proposed return of the boat and its crew would avoid a potential flashpoint to the months-long standoff between Pyongyang and South Korea and its US ally.

The vessel and its crew would be released in waters at the military boundary between the two Koreas, which are still technically at war after their 1950-53 conflict ended in a truce not a peace treaty.

It will happen hours after a visit on Friday morning by US Defence Secretary James Mattis to the truce village of Panmunjom inside the Demilitarised Zone (DMZ).

Mattis said America's goal was not to wage war with Pyongyang but to convince leader Kim Jong Un to give up his nuclear arsenal.

"North Korean provocations continue to threaten regional and global security despite unanimous condemnation by the United Nations Security Council," Mattis said in prepared remarks as he visited the DMZ.

Ahead of a visit by US President Donald Trump to Asia starting next week, Mattis has emphasised diplomatic efforts to find a peaceful solution to the current crisis during his week-long trip to the region.

"I carried the message that the more we do together today the greater the chance for enduring peace in the future," Mattis said earlier this week following three days of meetings with Asian defence chiefs in the Philippines.

"That's really what it was all about - to keep the (North Korea) effort firmly in the diplomatic lane for resolution."

"Do we have military options in defence for attack, if our allies are attacked? Of course we do. But everyone is out for a peaceful resolution," Mattis told reporters travelling with him earlier this week. "No one's rushing for war."

The highest-ranking military officers of South Korea and the United States on Friday (Oct 27) held talks in Seoul on key alliance issues and will report the outcome of their talks to the defence ministers -- South Korea's Song Young Moo and Mattis on Saturday.

In a show of force, the US has sent three aircraft carriers and their missile-armed escorts to the western Pacific Ocean for the first time since 2017 ahead of Mr Trump's visit to Asia.

"This was a unique opportunity to show that the US (is) the only power in the world that can demonstrate that kind of presence, and a unique opportunity for them to be together," chief Pentagon spokesperson Dana White said during a regular press briefing on Thursday.

"It's not directed towards any particular threat, but it's a demonstration that we can do something that no one else in the world can."

The deployment of USS Nimitz, USS Theodore Roosevelt and USS Ronald Reagan came amid heightened tensions over North Korea's nuclear and ballistic missile tests.

The United States on Thursday imposed sanctions on seven North Korean individuals and three entities for "flagrant" human rights abuses, including killings, torture, forced labour and the hunting down of asylum seekers abroad.

The South's Unification Ministry's spokesman Baik Tae Hyun said the North's message via its state agency early on Friday was the first contact Seoul had received regarding the vessel, Baik told a regular media briefing.

The fishing boat, which left port on Oct 16, had been reported as missing from Oct 21 and relevant authorities had been searching for the vessel, Baik added.

North Korea said it had captured the boat on Oct 21 and will release it at 5.30 pm on Friday (Singapore time) in waters off the east coast.

A report by North Korea's news agency KCNA said that an investigation by the North had proved the boat and crew had entered North Korean waters for fishing.

North Korea decided to release the boat after "taking into account the fact that all the crewmen honestly admitted their offence, repeatedly apologising and asking for leniency," the report said in English.

North Korean fishing boats have been found drifting south of the maritime border between the two Koreas at times, often having run out of fuel or broken down. Most North Korean crew are released to the North after interrogations by intelligence officials if they wish to return.

It is more unusual for South Korean fishing vessels to be found under similar circumstances.

A South Korean Unification Ministry official said it was aware the fishing boat had gone missing earlier in the week. The crew of 10, including seven South Koreans and three Vietnamese, would be questioned by officials on their return, he added.

The last time North Korea released a South Korean ship was in September 2010 roughly a month after a fishing vessel had accidentally drifted north of the maritime border following engine malfunctions.

Even before landing in Seoul on Friday, Mattis held a meeting in the Philippines on Monday with his South Korea and Japanese counterparts, where they agreed to keep bolstering intelligence sharing about North Korea and enhance exercises.

Japanese Defence Minister Itsunori Onodera warned the threat from North Korea has grown to a "critical and imminent level".

CIA chief Mike Pompeo said last week North Korea could be only months away from developing the ability to hit the United States with nuclear weapons, a scenario Trump has vowed to prevent.

US intelligence experts say Pyongyang believes it needs the weapons to ensure its survival and have been sceptical about diplomatic efforts, focusing on sanctions, to get Pyongyang to denuclearize.

Trump, in a speech last month at the United Nations, threatened to destroy North Korea if necessary to defend itself and allies. Kim has blasted Trump as "mentally deranged".

Despite the rhetoric, White House officials say Trump is looking for a peaceful resolution of the standoff. But all options, including military ones, are on the table.