North Korea has launched a number of missiles in recent days
Russia, China and the US have all called for calm after North Korea test-fired a series of missiles.
Seven Scud-type ballistic missiles with a range of about 500km (312 miles) were fired in an apparent act of defiance against the US, on 4 July.
Russia and China called on Pyongyang to return to talks, while a US official urged it not to aggravate tensions.
North Korea is banned from all ballistic missile-related activities under UN sanctions.
The sanctions were strengthened after the communist nation carried out a second underground nuclear test in May.
North Korea has launched a number of missiles since the test. On Thursday it fired four short-range missiles.
Saturday's launches came from a base on its east coast. Three were fired in the morning, a fourth around noon local time and three more in the afternoon, South Korea's Joint Chiefs of Staff said.
NORTH KOREA 2009 TESTS
4 July - Seven suspected ballistic missiles fired
2 July - four short-range cruise missiles launched
25 May - second underground nuclear test brings new UN sanctions
25/26 May - series of short-range rockets fired
5 April - N Korea says long-range rocket was satellite launch
The missiles fell into the Sea of Japan, known in South Korea as the East Sea.
South Korean officials said some of the missiles could have been medium-range Nodongs - which have the range to strike Japan - but that their flight distances had been deliberately shortened, Yonhap news agency said.
Both South Korea and Japan called the launches an "act of provocation".
A spokesman for the US state department called them "not helpful" and said North Korea should " refrain from actions that aggravate tensions and focus on denuclearisation talks".
Russia and China called on all parties to show restraint and avoid actions which could further destabilise the situation, the Russian Foreign Ministry said in a statement.
British Foreign Secretary David Miliband condemned the launches and said it was vital that "tension on the Korean peninsula [was] kept at manageable levels".
Ties between North Korea and the outside world have grown extremely tense since it walked away from six-nation talks aimed at ending its nuclear programme.
It subsequently said it would "weaponise" its plutonium stocks and start enriching uranium, prompting fears that it is working to produce nuclear warheads small enough to put on missiles - though analysts say it could take a long time to do so.
On 12 June the UN Security Council approved a resolution allowing inspection of air, sea and land shipments in and out of North Korea suspected of carrying banned arms and weapons-related material.
The North has said it will treat any interception of its ships as a declaration of war.
Some analysts suggest Pyongyang's bellicosity could be linked to internal issues, amid questions over the health of leader Kim Jong-il after his apparent stroke last year.
Reports suggest he has named his youngest son as his successor - and some believe the nuclear and missiles tests could be a show of strength aimed at securing internal support for his plans.