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North Korea fired two ballistic missiles on Friday, one of which flew about 800 kilometers (500 miles) while the other exploded shortly after launch, US officials said, as the isolated state stepped up its defiance of tough new UN and US sanctions.

US officials told Reuters the medium-range missiles appeared to be fired from road-mobile launchers.

One missile, fired from north of the capital, Pyongyang, flew across the peninsula and into the sea off the east coast early Friday morning, South Korea's Office of the Joint Chiefs of Staff said in a statement.

That would mark North Korea's first test of a medium-range missile, one of which was capable of reaching Japan, since 2014.

US officials, speaking on condition of anonymity, said the second missile flew a short period before exploding.

South Korea did not confirm the type of missile but US officials said they were medium-range ballistic missiles.

A range of 800 kilometers was likely beyond the capability of most short-range missiles in North Korea's arsenal. The North's Rodong missile has an estimated maximum range of 1,300 kilometers (810 miles), according to the South's Defense Ministry.

North Korea's action provoked a barrage of criticism and appeals.

Chinese Foreign Ministry spokesman Lu Kang urged it to abide by UN resolutions and not do anything to exacerbate tensions.

US Defense Secretary Ash Carter said the United States was "analyzing the results of those launches," but called on Beijing to use its influence over Pyongyang.

"China could do a lot more," Carter said, adding Beijing should seek a nuclear-free North Korea.

The US State Department in a statement urged North Korea to focus on taking concrete steps toward fulfilling its international commitments and obligations.

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Japan lodged a protest with North Korea through its embassy in Beijing, Prime Minister Shinzo Abe told parliament.

"Japan strongly demands North Korea to exercise self-restraint and will take all necessary measures, such as warning and surveillance activity, to be able to respond to any situations," Abe said.

South Korea's Unification Ministry said Pyongyang should focus on improving the lives of its people and that provocative actions would help nothing.

North Korea often fires missiles during periods of tension on the Korean peninsula or when it comes under pressure to curb its defiance and abandon its weapons programs.

Last week, the North fired two short-range missiles into the sea off its east coast and its leader Kim Jong-un ordered more nuclear weapons tests and missile tests.

That came after North Korean media said the North had miniaturized nuclear warheads to fit on ballistic missiles and quoted Kim as calling on the military to prepare for a "pre-emptive nuclear strike" against the United States and South Korea.

US President Barack Obama imposed new sanctions on North Korea on Wednesday over its nuclear test and satellite launch. The sanctions freeze North Korean government assets in the United States, bans US exports to, or investment in, North Korea, and expands a US blacklist to anyone, including non-Americans, who deals with North Korea.

North Korea conducted its fourth nuclear test on January 6 and launched a long-range rocket on February 7 in defiance of existing UN Security Council resolutions.

The North has called annual joint drills by US and South Korean troops that began on March 7 "nuclear war moves" and threatened to wipe out its enemies.

The US and South Korea remain technically at war with the North because the 1950-53 Korean War ended in an armed truce instead of a peace agreement. In recent weeks, the two Koreas have suspended economic ties over the mounting tensions.

South Korea and US officials this month began discussions on deploying the advanced anti-missile Terminal High Altitude Area Defence (THAAD) system to the US military in the South, despite Chinese and Russian objections.

On Wednesday, North Korea's supreme court sentenced a visiting American student to 15 years of hard labor for crimes against the state, a punishment Washington condemned as politically motivated.