South Koreans at the Seoul Railway Station in the South Korean capital on Saturday view a televised but undated image of a missile launch by North Korea. (Ahn Young-joon/associated Press)

North Korea has launched a series of missiles off its eastern coast, following similar tests earlier in the week, according to South Korea's Defence Ministry.

Pyongyang fired seven ballistic missiles over several hours Saturday, the Yonhap news agency said, quoting military officials.

The South Korean agency said it appears the North fired a type of short-range Scud missile.

But Yonhap also said it is possible they could have been longer-range Rodong missiles.

South Korea's Joint Chiefs of Staff said the missiles are believed to have flown more than 400 kilometres.

"Our military is fully ready to counter any North Korean threats and provocations based on strong South Korea-U.S. combined defence posture," the Joint Chiefs of Staff said in a statement.

North Korea is not allowed to fire Scuds, medium-range missiles or long-range missiles. They are banned under UN Resolution 1874, passed after North Korea's May 25 underground nuclear test, prohibiting any launch using ballistic missile technology.

On Thursday, North Korea fired four short-range missiles. Those launches did not violate the resolution as they were cruise missiles rather than ballistic, according to South Korea's Foreign Ministry.

Ballistic missiles are guided during their ascent out of the atmosphere but fall freely when they descend. Cruise missiles fly low and straight to their target.

Three years ago on July 4, North Korea launched a long-range Taepodong-2 multi-stage ballistic missile and six smaller missiles in defiance of a UN resolution.

With files from The Associated Press