The United States and South Korea have begun annual military drills, amid high tensions with North Korea in the wake of a United Nations sanctions vote.
Pyongyang has strongly condemned the exercises, threatening to scrap the armistice that ended the Korean War.
Seoul says North Korea also appears to have carried out a threat made last week to sever a cross-border hotline, the BBC reports.
The drills come days after the UN approved new sanctions on North Korea following its nuclear test in February.
The test last month was the communist country's third.
It followed an apparently successful launch in December of a three-stage rocket, seen as a banned test of missile technology.
North Korea's neighbours and the US fear it is working to build a nuclear warhead small enough to put on a missile, but believe it does not yet have the capabilities to do so.