North Korea fired several short-range projectiles into the sea on Thursday, hours after the United Nations' Security Council voted to impose tough new sanctions on the isolated state and South Korean President Park Geun-hye vowed to "end tyranny" by the North's leader.
The firing escalated tensions on the Korean peninsula, which have been high since the North's January nuclear test and February long-range rocket launch, and set the South's military on a heightened alert.
South Korea's Defence Ministry said it was trying to determine if the projectiles, launched at 10 am from the North's east coast, were short-range missiles or artillery fire.
Park has been tough in her response to the North's recent actions, moving from her earlier self-described "trustpolitik" approach, and on Thursday welcomed the move by the Security Council and repeated her call for the North to change its behaviour.
"We will cooperate with the world to make the North Korean regime abandon its reckless nuclear development and end tyranny that oppresses freedom and human rights of our brethren in the North," Park said at a Christian prayer meeting on Thursday.
Also on Thursday, South Korea adopted a long-delayed security law to set up an anti-espionage unit that was passed by parliament late on Wednesday, and another law aimed at improving human rights in North Korea.
Last month, Seoul suspended the operation of a jointly run factory project with the North that had been the rivals' last remaining venue for regular interaction.
In its latest barrage of insults against the South's leader, the North's official media carried a commentary on Wednesday likening Park to an "ugly female bat", fated to "die in the dreary cave, its body hanging down".
North Korea faces harsh new sanctions for its nuclear weapons programme under the resolution passed unanimously by the Security Council on Wednesday, drafted by the United States and backed by the North's main ally China.
The resolution, which dramatically expands existing sanctions, follows the North's fourth nuclear test on January 6 and a long-range rocket launch on February 7, which the United States and South Korea said violated existing Security Council resolutions.
The North says it was its sovereign right to launch rockets as part of a space programme to put satellites into orbit.