SEOUL - Mounting pressure from the US could just be the impetus South Korea needs to reverse its decision to end a military intelligence sharing pact with Japan ahead of a looming deadline, experts say.

If not, the optics would be bad for South Korea and the United States, which had pushed for the 2016 General Security of Military Information Agreement (GSOMIA) to strengthen trilateral cooperation with its allies and keep nuclear-armed North Korea in check.

The deal is due to expire on Nov 23, after Seoul announced in August that it would not seek renewal amid soured bilateral ties with Tokyo over a trade spat.

Various US officials have urged South Korea and Japan to renew the agreement.

Defence Secretary Mark Esper weighed in on Friday (Nov 15), underscoring the fact that GSOMIA will be an "important tool" for the US and its two allies to share information effectively, especially in times of war.

"The only ones who will benefit from expiration of GSOMIA and continued friction between Seoul and Tokyo are Pyongyang and Beijing," he said in Seoul.

"That reason alone should be powerful enough for us to... make sure we restore our alliance to where it was, so we can work together to respond to our common threats and challenges."

Allowing the pact to lapse could also "present unnecessary opportunity costs to future security cooperation", according to International Studies Professor Leif-Eric Easley of Ewha Womans University.

"The optics would be bad for South Korea and for US alliances, suggesting an erosion in the multilateral coordination that is important for deterrence and defending a rules-based order in Asia," he told The Straits Times.

It is not too late for the administration of President Moon Jae-in to renew GSOMIA, he added.

"The face-saving way to do this is to cite US pressure and explain that Seoul needs to focus efforts on (defence) cost-sharing negotiations with Washington rather than the politics of intelligence-sharing with Tokyo," he said.

"The Moon administration can claim that its signals about the need to resolve Japan's export controls have been received, but a November deadline is too early and artificial for the process of rebuilding trust."

South Korea has reiterated that it is open to reconsideration, provided that Japan makes the first move to change its course of action.

Seoul views Tokyo's export restrictions and subsequent removal of South Korea from a white list of preferred trading partners as retaliation against a dispute over wartime forced labour.

Japan has refused to relent, citing security concerns and a lack of trust in North-friendly South Korea. It has asked South Korea for a "sensible response" on its decision to end GSOMIA.

During his meeting with Mr Esper on Friday, President Moon said it is difficult to share military information with Japan, which does not view Seoul as a trustworthy partner, according to a presidential spokesman.

But Mr Moon acknowledged the importance of a US-Japan-Korea security partnership and reaffirmed his commitment to it.

Dr Bong Young-shik of Yonsei University believes that Seoul should renew the pact, which will allow the government to "demonstrate willingness and the right attitude to make corrections deemed necessary" and show that dialogue and coordination can help iron out differences in the management of a security partnership.

Not doing so will "frustrate the Trump administration and South Korea will lose credibility as a US security partner", he added.

While GSOMIA was a top priority of Mr Esper's visit to Seoul, the Pentagon chief also broached other issues such as defence cost sharing and joint military exercises.

He pressed South Korea to pay more to maintain 28,000 American troops in the country.

Negotiations are under way to strike a new cost-sharing agreement. Seoul currently pays US$870 million (S$1.2 billion) a year, but Washington has reportedly demanded nearly US$5 billion for the next year.

Mr Esper also called for flexibility on joint military exercises that irk North Korea, which is in talks with the US to end its nuclear programme.

"The purpose of our forces and exercises is not only to buttress our diplomacy, but to also enable and empower it... (We have to) ensure that we do not close any doors that may allow forward progress on the diplomatic front."