WASHINGTON -- The top nuclear envoys of South Korea and the United States met in Washington on Thursday, a diplomatic source said, after North Korea threatened to take military action against the South.

Lee Do-hoon, special representative for Korean Peninsula peace and security affairs, and his US counterpart, Deputy Secretary of State Stephen Biegun, held talks at an undisclosed location earlier in the day, the source said on condition of anonymity.

Their meeting came as tensions are running high on the Korean Peninsula after North Korea blew up an inter-Korean liaison office and threatened to redeploy troops to border areas to protest the sending of propaganda leaflets by defectors into the North.

Lee arrived in the US capital unannounced Wednesday, leading to speculation that he may have been sent as a special envoy by the presidential office, Cheong Wa Dae.

He refused to answer reporters' questions about the purpose of his visit or whom he will be meeting.

Possible interlocutors were thought to include National Security Adviser Robert O'Brien and Biegun, who doubles as the US special representative for North Korea.

The State Department has expressed its disappointment at North Korea's recent actions to sever ties with the South and urged the communist country to refrain from "further counterproductive actions."