North Korea appointed a new prime minister Monday, choosing a former premier who was sacked from the post in 2007 in a reported backlash against his pursuit of economic reforms.

Pak Pong-Ju, 74, was sworn in at the annual meeting of the Supreme People's Assembly, the North's rubber-stamp parliament, the official Korean Central News Agency (KCNA) said. He replaces Choe Yong-Rim.

North Korea likely took meticulous steps to conceal any residue from its February nuclear weapon test, fueling suspicions that it is using a new bomb design with highly enriched uranium at its core, The Washington Post reported Sunday.

Citing unnamed U.S. officials and weapons experts, the newspaper said the effects of the February 12 explosion were remarkably well contained, with few radioactive traces escaping into the atmosphere.

The U.S. military said Monday it had deployed stealth fighters to South Korea as part of an ongoing joint military exercise that has triggered dire North Korean threats of armed retribution.

Two F-22 Raptor fighters arrived in the South on Sunday to participate in the annual "Foal Eagle" exercise that will last until April 30, a spokesman for the U.S. forces in South Korea told Agence France Presse.

North Korea vowed Sunday to strengthen its nuclear weapons, a day after announcing it is in a "state of war" with South Korea, and said it would never trade its atomic deterrent for aid.

Tensions have risen sharply since the United Nations tightened sanctions in response to the North's nuclear and missile tests, and since the United States and South Korea launched military drills south of the border.

Pope Francis prayed for "reconciliation" on the Korean peninsula in his first Easter Sunday message in front of a crowd in St Peter's Square.

Latin America's first pontiff also issued an appeal for hostages held by militants in Nigeria and condemned human trafficking as "the most extensive form of slavery in this 21st century".

North Korea on Saturday declared it had entered into a "state of war" with South Korea and warned Seoul and Washington that any provocation would swiftly escalate into an all-out nuclear conflict.

The United States said it took the announcement "seriously,” even though it followed a familiar pattern, while South Korea largely dismissed it as an old threat dressed in new clothing.

Russia warned Friday that a flare-up in tensions between North Korea and the United States could spin out of control, urging all sides involved in the standoff to refrain from muscle-flexing.

"Unilateral actions are being taken around North Korea which manifest themselves in an escalation of military activity," Russian Foreign Minister Sergei Lavrov said.

Tens of thousands of North Korean soldiers and civilians held a huge rally and march in Pyongyang on Friday, in a mass display of support for a possible military strike against the United States.

The rally in Pyongyang's giant Kim Il-Sung square was attended by soldiers, veterans, workers and students, all wearing military uniforms. The North's young leader, Kim Jong-Un, was not present.

China appealed for "joint efforts" to reduce tensions on the Korean peninsula Friday, after Pyongyang ordered missile units to prepare to strike the U.S. mainland and U.S. stealth bombers flew over South Korea.

"We call on all relevant parties to make joint efforts to turn around the tense situation," foreign ministry spokesman Hong Lei told a regular press briefing in Beijing.

Iran, North Korea and Syria blocked adoption of a U.N. treaty that would regulate the multibillion-dollar international arms trade for the first time, saying it fails to ban sales to terrorists, but other countries refused to let the treaty die.

The treaty's adoption required agreement by all 193 U.N. member states, but some countries said Thursday they would ask Secretary-General Ban Ki-moon to bring the final draft before the General Assembly for adoption by vote as soon as possible. Observers said that could be as soon as Tuesday.