A surge in activity at North Korea's atomic test site suggests preparations for a fifth nuclear test are in their final stages, possibly before a key political event in early May, South Korean media reported Sunday.

The frequency of vehicle, workforce and equipment movements at the Punggye-ri site have "increased two to threefold," since last month, Yonhap news agency said, citing multiple government sources.

Officials believe the trucks seen moving in and out of the complex are likely carrying nuclear technicians.

"If they are signs of nuclear test preparations, it seems the preparations are in the final stages," Yonhap quoted one government source as saying.

North Korea is gearing up for a rare and much-hyped ruling party congress early next month, at which leader Kim Jong-Un is expected to take credit for pushing the country's nuclear weapons programme to new heights.

Numerous analysts have suggested the regime might carry out a fifth nuclear test as a display of defiance and strength just before the congress opens.

A successful test might also go some way to erasing the embarrassing failure on Friday of a medium-range ballistic missile test meant to mark the birthday of the nation's founder Kim Il-Sung.

Pyongyang has claimed a series of achievements in recent months, including miniaturising a nuclear warhead to fit on a missile, developing a warhead that can withstand atmospheric re-entry, and building a solid-fuel missile engine.

Tension has been running high on the divided peninsula since the North conducted its fourth nuclear test in January and a rocket launch a month later that was widely seen as a disguised ballistic missile test.

The UN Security Council responded with its toughest sanctions to date, angering the North. It has since made repeated threats of attacks targeting Seoul and Washington.

N. Korea claims restaurant workers tricked into defecting
Seoul (AFP) April 15, 2016 - Pyongyang's state media Friday claimed a group of 13 North Koreans working at a restaurant China had been tricked into defecting by South Korean spies.

The 13 -- a male manager and 12 female employees of a Pyongyang-run restaurant -- arrived in Seoul last week, South Korea's Unification Ministry said.

Ministry spokesman Jeong Joon-Hee said they had been under pressure to meet their quota for hard currency remittances to Pyongyang and had become disillusioned with the North's propaganda after being exposed to foreign media.

The South Korean government estimates Pyongyang rakes in around $10 million every year from some 130 restaurants it operates -- with mostly North Korean staff -- in 12 countries, including neighbouring China.

North Korea's propaganda website Uriminzokkiri claimed that the manager, who it said was bribed by South Korean spies, had tricked the 12 others into thinking they were going to a new workplace.

"The manager, in connivance with (South Korean) spies, tricked the employees into believing that they were moving to a different workplace in a far-off place" before putting them on a plane, it said in a commentary.

It said the "abduction" was part of the South's "heinous" anti-Pyongyang smear campaign.

South Korean daily Hankyoreh Sinmun quoted the Chinese owner of the restaurant in Ningbo, in eastern Zhejiang province, as saying the manager had stolen more than 1.2 million renminbi ($185,000) from the restaurant, which had a total of 20 North Korean employees.

The seven remaining North Koreans were staying with North Korean embassy officials and were expected to return home, the owner was quoted as telling the paper.

The 13 defectors were still incommunicado, being questioned by South Korean authorities.

The North's Red Cross spokesman Tuesday claimed the 13 were kidnapped by the South and demanded it apologise and return them immediately or face "unimaginable consequences and strong countermeasures".

Separately, the North Korean website also accused Seoul of trying to influence South Korea's parliamentary elections held on Wednesday.

A seemingly unstable North Korea has traditionally worked in favour of the ruling conservative party.

But President Park Geun-Hye's ruling Saenuri Party suffered a shock electoral defeat that broke its 16-year parliamentary majority and threatened its chances of retaining the presidential Blue House in 2017.