Russia has warned of tensions in North Korea slipping out of control, after Pyongyang said it was placing its missile units on stand-by.

Russian Foreign Minister Sergei Lavrov warned the situation could slip "toward the spiral of a vicious circle".

Kim Jong-un made the missile order after talks responding to US stealth bomber flights over the Korean peninsula, state news agency KCNA said.

The time had come to "settle accounts" with the US, KCNA quoted him as saying.