North Korea is thought to have thousands of missiles
North Korea has test-fired a series of missiles in an apparent act of defiance on 4 July, American Independence Day.
Reports say at least seven Scud-type ballistic missiles were fired, with a range of about 500km (312 miles).
South Korea and Japan called the latest tests, which follow several others in recent weeks, an "act of provocation".
North Korea is banned from all ballistic missile-related activities under UN sanctions imposed after a second underground nuclear test in May.
South Korea's Joint Chiefs of Staff (JCS) said the missiles were fired from one of the North's east coast launch sites on Saturday morning.
All landed in the Sea of Japan, known in South Korea as the East Sea.
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
"Our military is fully ready to counter any North Korean threats and provocations," the JCS said in a statement.
A South Korean defence official said Saturday's tests were of greater concern than four short-range ones on Thursday, as the missiles had longer ranges.
"Thursday's missile tests were apparently made as part of a military drill but today's launches, which came on the eve of the US Independence Day, are believed to be aimed at political purposes," the official told Yonhap news agency.
The South Korean government condemned the launches for being in breach of the recent UN security council resolution.
A spokesman for the US state department, meanwhile, described North Korea's behaviour as "not helpful" and urged it not to aggravate tensions, AFP news agency reported.
Nuclear warhead fears
The BBC's John Sudworth in Seoul says the launches are seen there as part of North Korean efforts to ratchet up the tension.
The missiles' estimated 500km reach - although still technically short-range - brings most of South Korea withing striking distance, our correspondent says.
Japanese and South Korean media have reported that North Korea may be preparing to launch an intercontinental ballistic missile.
Our correspondent says there are no signs that such a test could be imminent.
Pyongyang is banned from testing ballistic missiles under UN resolutions but launched a long-range rocket in April, which many governments saw as a thinly disguised test of Taepodong-2 missile technology.
There are fears that North Korea is trying to produce nuclear warheads small enough to put on missiles - but experts say this appears to be several years away.
After six-nation talks aimed at curbing North Korea's nuclear ambitions broke down earlier this year, Pyongyang said it would "weaponise" its plutonium stocks and start enriching uranium.
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.
Korea analyst Aidan Foster-Carter told the BBC that the international community will have to wait and see what North Korea wants, while reprimanding it and taking what action it can.
He believes that the political situation inside North Korea could be behind its recent behaviour.
There are 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 analysts believe the recent bellicosity could be a show of strength aimed at securing domestic support for his plans.