SEOUL (AFP) - South Korea's national security adviser has flown to Washington to meet his newly appointed US counterpart John Bolton, an official said on Thursday (Apr 12) in Seoul, where concerns are growing over the American's hardline stance on the nuclear-armed North.

Mr Chung Eui Yong, who personally delivered North Korean leader Kim Jong Un's summit offer to US President Donald Trump last month, returned to the US capital unannounced on Wednesday.

The two allies and are planning their upcoming summits with Mr Kim separately.

Mr Chung is due to meet Mr Bolton on Thursday, the official at the South's presidential Blue House told reporters.

He added that the meeting was a courtesy call aimed at "coordinating opinions ahead of the inter-Korean and US-North Korea summits".

The rare inter-Korean summit is scheduled to take place in just over two weeks, while the second meeting between Mr Trump and Mr Kim is expected as early as May.

The appointment this week of Mr Bolton - a former United Nations ambassador and strong defender of a US first-strike option against North Korea - has raised concerns in Seoul over the prospects of the diplomatic thaw on the Korean peninsula.

Mr Bolton's stance on Pyongyang diverges sharply from the more dovish South Korean government, which had pushed hard for an early meeting with him, the South's Yonhap agency reported.

"Bolton does not even mention the possibility of a peace treaty with Pyongyang" which is sought by the South, said Professor Koo Kab Woo at the University of North Korean Studies in Seoul.

"If Bolton intervenes in the current diplomatic process (with North Korea), the need for coordination becomes much bigger," Prof Koo added.

In a growing rapprochement on the Korean peninsula kickstarted by the South's Winter Olympics, Mr Kim is set to meet South Korean President Moon Jae In on Apr 27.

Mr Trump has agreed to meet Mr Kim to discuss denuclearisation as soon as next month, though no venue or specific date is agreed.

The South Korean embassy in Washington and the US State Department have agreed to set up a hotline in the build-up to the summits, Yonhap reported.

The South's presidential office is also set to launch a new situation room to monitor the preparations for its upcoming summit on a daily basis, it said on Thursday.