Representatives of the world's five big nuclear-armed states on Friday voiced concern over North Korea and Iran's nuclear programs, lamenting the threat to global efforts to avoid proliferation.

"In the context of the nuclear test conducted by (North Korea) on February 12, 2013, and the continued pursuit of certain nuclear activities by Iran ... the P5 reaffirmed their concerns about these serious challenges to the non-proliferation regime," they said in a statement.

North Korea on Friday rejected a new request by businessmen from the South to deliver food and supplies to their staff inside a closed joint-industrial zone, officials said.

North Korea has blocked access to the zone in Kaesong -- which lies 10 kilometers (six miles) inside its border -- since April 3 amid soaring military tensions on the Korean peninsula.

North Korea laid out onerous conditions Thursday for any talks with Seoul or Washington, including the withdrawal of U.N. sanctions and a guaranteed end to South Korea-U.S. joint military drills.

"If the U.S. and the South enemies... genuinely want dialogue and negotiation, they should take these steps," the North's National Defense Commission said in a statement.

North Korea on Wednesday barred a delegation of South Korean businessmen from delivering food and supplies to 200 of their staff inside the closed Kaesong joint industrial zone.

Ten representatives of the 123 South Korean firms in Kaesong had applied for permission to visit the zone, two weeks after the North blocked all access amid soaring military tensions on the Korean peninsula.

A U.S. military helicopter taking part in a joint South Korea-U.S. drill crashed Tuesday near the North Korean border with no apparent casualties, officials said.

A U.S. military official identified the aircraft as a CH-53 U.S. Marine helicopter that was carrying three crew and 13 other personnel during a training exercise.

North Korea's military on Tuesday threatened the South with imminent "sledge-hammer" retaliation unless Seoul apologized for anti-Pyongyang protestors burning effigies of its revered leaders.

South Korea called the North's ultimatum "regrettable" and vowed a tough response to any military provocation.

U.S. Secretary of State John Kerry pushed a message of dialogue with North Korea at the end of an Asia tour on Monday, as Pyongyang kept the world guessing over an expected missile launch.

The North has a habit of linking high-profile military tests with key dates, and expectations had been high of a mid-range missile test to coincide with Monday's celebrations marking the birth of its late founding leader Kim Il-Sung.

South Korea voiced regret Monday at North Korea's dismissal of its offer for dialogue, as the South's armed forces remained on heightened alert for an expected missile test by Pyongyang.

Both President Park Geun-Hye and her Unification Ministry have, in recent days, made tentative proposals for talks in an apparent bid to reduce soaring military tensions on the Korean peninsula.

North Korea dismissed on Sunday the South's offer for dialogue on the future of the Kaesong joint industrial zone, calling the offer an "empty" political gesture.

The North announced the withdrawal of its 53,000 workers and the suspension of operations at Kaesong at the beginning of the week, as military tensions on the Korean peninsula soared.

U.S. Secretary of State John Kerry arrived in Japan on Sunday to discuss nuclear tensions on the Korean peninsula after securing vital support from China to help defuse the weeks-long crisis.

He was due to meet Japanese Foreign Minister Fumio Kishida later in Tokyo, which has deployed Patriot missiles around the capital in anticipation of a missile launch by the North.