By Jon Herskovitz

SEOUL (Reuters) - North Korea fired six ballistic missiles on Saturday, South Korea's defense ministry said, in an act of defiance toward the United States that further stoked regional tensions already high due to its nuclear test in May.

South Korea's Yonhap news agency said the missiles test-fired were "Scud-type," marking an escalation of recent saber-rattling by the reclusive North, which has fired several non-ballistic, short-range missile since the May 25 nuclear test.

North Korea is barred by United Nations resolutions from firing ballistic missiles such as the Scud.

South Korea's Defense Ministry confirmed the North fired six missiles off its east coast over a period of around eight hours that traveled for about 400 km and splashed into the sea.

"It is a provocative act that clearly violates U.N. Security Council resolutions," Yonhap quoted a statement from South Korea's Foreign Ministry as saying.

"The government expresses deep regret over North Korea's continued acts to escalate tensions in Northeast Asia."

North Korea is thought to have more than 600 Scud-type missiles that include the Hwasong-5, with a range of about 300 km (185 miles) and the Hwasong-6, with a range of about 500 km (310 miles).

Japan, a party to currently suspended six-nation talks aimed at coaxing the isolated North to give up its nuclear programme in return for aid and greater diplomatic recognition, was also quick to condemn Pyongyang's latest action.

"Japan strongly protests and regrets today's missile launches by North Korea as they are a serious act of provocation against the security of neighboring countries, including Japan, and is against the resolution of the U.N. Security Council," Japan's foreign ministry said in a statement.

North Korea fired a barrage of four short-range, non-ballistic missiles into the sea off its east coast on Thursday.

FOURTH OF JULY

In 2006, North Korea test-fired its long range Taepodong-2 missile and other ballistic missiles while the United States celebrated its July 4 Independence Day national holiday.

An anonymous South Korean official quoted by Yonhap said the North's latest move appeared to be intended to send a message to the United States.

"The missiles fired on July 2 were analyzed to be part of military drills, but today's missiles seem to have political purposes in that they were fired a day ahead of the U.S. Independence Day," the agency quoted the official as saying.

U.N. sanctions put in place after the North's nuclear test in May were aimed at halting its trade in missiles systems, which are a vital source of foreign currency for the cash-short state.

The U.S. envoy who coordinates sanctions against the North, was in China earlier this week to enlist Beijing's help in getting tougher with North Korea.

China is the North's biggest benefactor and trade partner whose help would be essential for an effective sanctions regime, analysts said.

Daniel Pinkston, with the International Crisis Group in Seoul, said the test helps the North's military in its missile capabilities and could also be linked to the sanctions.

"The sanctions raised the cost of products such as missile systems. Buyers, who are taking increased risks, want to be assured about the quality and reliability of the product," said Pinkston.

North Korea fired a rocket it said put a satellite into space in April. U.S., South Korean and other officials said the launch was a disguised test of the long-range Taepodong-2 missile, which could hit U.S. territory, and nothing was put into orbit.

The North has raised tension in recent months by saying it has started a programme to enrich uranium, which could give it a second path to a nuclear bomb, threatening to attack the South, and extracting plutonium at its aging Yongbyon nuclear plant.

Analysts said the moves may be aimed at securing internal support for leader Kim Jong-il, 67 and thought to have suffered a stroke a year ago, as he prepares the ground for his youngest son to succeed him at the head of Asia's only communist dynasty.

(Additional reporting by Aiko Hayashi in Tokyo and Seo Eun-kyung in Seoul; Editing by Alex Richardson)