A Japanese journalist on trial for allegedly defaming South Korean President Park Geun-hye said on Thursday that a ban on his leaving the country had been extended by another three months.

The case has strained relations between Seoul and Tokyo, which condemned the extension as a "grave humanitarian issue" and said it would issue a formal protest.

"I was told by my lawyers that the ban on my leaving the country had been renewed for three more months," Tatsuya Kato said.