US President Donald Trump says he has given South Korea approval to build a nuclear-powered submarine, a dramatic move that would admit Seoul to a small club of nations possessing such vessels.

The submarine will be built in a Philadelphia shipyard where South Korean firms have increased investment, Trump wrote on social media on Thursday.

The US president, who has been meeting with South Korean President Lee Jae Myung and other regional leaders during his visit, also said Seoul had agreed to buy vast quantities of US oil and gas.

Trump and Lee finalised details of a fraught trade deal at a summit in South Korea on Wednesday.

Lee had also been seeking US permission for South Korea to reprocess nuclear fuel.

Seoul is barred from reprocessing without US consent, under a pact between the

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