North Korea has test-fired four short-range missiles, South Korea's Yonhap news agency has reported.
The agency quoted defence officials as saying the fourth was launched at around 1220GMT from Sinsang-ni.
Three surface-to-ship missiles had earlier been fired - two near the port of Wonsan and one more from Sinsang-ni.
The UN tightened sanctions against North Korea after it launched several missiles and carried out an underground nuclear test in May.
The first two missiles were fired from a base near Wonsan, and the third and fourth from nearby Sinsang-ni, the South Korean defence ministry said.
South Korean defence minister Won Tae-jae said the missiles were "surface-to-ship" ones and had been "fired into the East Sea", reported Yonhap.
South Korean media had reported that a missile launch was imminent. Previous North Korean warnings to shipping to avoid its coastal waters have been preludes to test launches.
Japanese and South Korean media have also reported that North Korea may be preparing to launch a long-range ballistic missile.
Such a rocket was launched in May, with Pyongyang saying it was intended to put a satellite into orbit.
There are fears that North Korea is trying to produce nuclear warheads small enough to put on long-range missiles.
The North may be about to launch another long-range missile, media say
After six-nation talks aimed at ending North Korea's nuclear ambitions broke down earlier this year, Pyongyang said it would "weaponise" its plutonium stocks and start enriching uranium for a light-water nuclear reactor.
Enriched uranium can be used as fuel for a nuclear reactor or, if more-highly enriched, it can be used to make atomic bombs.
Earlier this week, a North Korean ship suspected of transporting weapons to Burma turned around and appeared to head back to North Korea.
It has been under US surveillance for about a week and is the first vessel monitored under the new UN sanctions.
Resolution 1874, passed on 12 June, allows for inspections of air, sea and land shipments in and out of North Korea suspected to be carrying banned arms and weapons-related material.
The North has said it will treat any interception of its ships as a declaration of war.
The latest missile launches came hours after North and South Korean officials ended talks at the joint Kaesong industrial zone in North Korea without progress being made.
South Korean officials said they had rejected a Northern demand, made in earlier meetings, to pay higher wages and increased rent in the factory zone, located just across the border in the North.