North Korea has fired a short-range missile into the sea and tried to jam GPS navigation signals in South Korea, officials in Seoul said.
The move came hours after US, South Korean and Japanese leaders pledged to work closer together to prevent North Korea from advancing its nuclear and missile programmes.
Officials said the attempt to jam GPS signals, which began on Thursday, did not cause any major disruptions of South Korean military, aviation and sea transport and telecommunication systems.
However, more than 130 fishing boats reported problems with their navigation systems and some were forced to return to port, the South Korean ministry of oceans and fisheries said.
The defence ministry called the jamming attempt a provocation that threatened public safety and military operations in the South. A statement warned North Korea to immediately stop the jamming or face unspecified consequences.
South Korea has blamed the North for several previous jamming attempts. This week's move to block signals are the first since 2012, according to South Korea's science ministry. No response was immediately forthcoming from North Korean state media.
North Korea also fired a surface-to-air missile off its east coast, three days after it launched a projectile that hit land in its north-east, South Korea's Joint Chiefs of Staff said.
The North has launched a number of short-range missiles and other projectiles since the start of annual South Korea-US military drills last month, which it views as a rehearsal for an invasion. It has also repeated threats of nuclear strikes on Seoul and Washington and warned it will test a nuclear warhead and the ballistic missiles capable of carrying it.
This year's drills, set to run until late this month, are the biggest ever and come after North Korea conducted a nuclear test and long-range rocket launch earlier this year.
In Washington, US president Barack Obama met the leaders of South Korea and Japan to discuss ways of countering North Korea's nuclear threat.
Mr Obama also met Chinese president Xi Jinping, with both calling for North Korea to give up its nuclear weapons.
China also agreed to fully implement recent economic restrictions imposed by the UN Security Council against North Korea.
The Asian leaders are in Washington for a two-day nuclear summit.