The North Korean army has declared invalid the armistice agreement that ended the Korean War in 1953, the official newspaper of the country's ruling Workers' Party said Monday.

Since last week, North Korea had been threatening to scrap the armistice after the U.N. Security Council passed tougher sanctions against it in response to its February 12 nuclear test.

On Monday, the Rodong Sinmun newspaper reported that the Supreme Command of North Korea's army had done so.

"The U.S. has reduced the armistice agreement to a dead paper," the newspaper said.

North Korea also cut off direct phone links with South Korea at the inter-Korean border village of Panmunjom, according to South Korea's Yonhap news agency. The phone line was the emergency link for quick, two-way communication between the two sides.

The armistice agreement, signed in 1953, ended the three-year war between North and South Korea in a truce.

Since the two sides remain technically at war, it remains to be seen whether the invalidation means that either side can resume hostilities.

The Rodong Sinmun reported the Supreme Command saying that it can now make a "strike of justice at any target anytime, not bound to the armistice agreement and achieve the national reunification, the cherished desire of the Korean nation."

However, the North has nullified the agreement on several occasions in the past.

Here's a look back at the history of the armistice.

What is the armistice agreement?

It is the agreement that ended the war between North and South Korea. It is a truce, rather than a peace treaty.

Has the North ended the armistice before?

Yes. In 2003, Pyonyang's official Korean Central News Agency announced that it may have "no option" but to stop honoring the armistice because of the United State's "persistent war moves."

In 2009, North Korea said its military would no longer be bound by the agreement because South Korea was joining a U.S.-led anti-proliferation plan.

Part of the reason for the latest move are the joint exercises between the United States and South Korea. A bigger reason is tougher sanctions passed in the U.N. Security Council against North Korea in response to its nuclear test on Feb. 12.

Pyongyang carried out its third nuclear test, despite international condemnation.

What caused the division of Korea?

For most of the first half of the 20th century, Japan controlled the Korean peninsula as its colony. By the end of the World War II as Japan neared defeat, the allies agreed to an independent Korea. The United States and Soviet Union divided postwar occupation of Korea along the 38th parallel and the two sides were ideologically opposite.

Why did war break out?