The country has been punished by the UN for its recent nuclear rocket test
North Korea has fired a number of short-range missiles into the Sea of Japan just hours after being hit with fresh UN sanctions.
A spokesman for South Korea's defence ministry said: "The South Korean military is monitoring any additional movements from the North."
Earlier the UN adopted its toughest sanctions to date in response to Pyongyang's fourth nuclear test and rocket launch.
The Security Council unanimously passed a resolution imposing new sanctions after weeks of negotiations between the US and China - North Korea's sole major ally.
The sanctions require all countries to inspect cargo destined for and coming from North Korea, in all airports and sea ports.
They also ban or restrict exports of coal, iron and iron ore and other minerals from North Korea, and prohibit the supply of aviation fuel including rocket fuel.
US President Barack Obama welcomed the measures as "a firm, united, and appropriate response" to the 6 January nuclear test and 7 February rocket launch.