The United States and its allies warned North Korea on Monday against provocations as researchers reported potential radiation risks due to problems at the regime's main nuclear complex.

The United States, South Korea and Japan, meeting in Washington after a new period of tension, condemned North Korea's recent ballistic missile tests and called again for an end to the regime's nuclear weapons program.

The three nations "urged the DPRK to refrain from further threatening actions," said a US statement, referring to the North by its official name of the Democratic People's Republic of Korea.

South Korea has been on guard after North Korea's young leader Kim Jong-Un warned last week of a "very grave" situation on the divided peninsula as he accused Seoul and Washington of trampling peace gestures through joint exercises.

In recent weeks, North Korea has test-fired medium-range ballistic missiles capable of hitting Japan, conducted a live-fire drill along its disputed border with South Korea and apparently flew three rudimentary drones over the border to peer at Seoul's military facilities.

- Fears for reactor -

A US think tank, reviewing recent satellite images, said Monday that North Korea's main Yongbyon nuclear site appeared to have suffered water supply problems due to heavy rain and floods last summer.

An unstable supply could pose radiation risks, especially at North Korea's first light water reactor, which is near completion, according to the US-Korea Institute at Johns Hopkins University.

The regime does not have experience operating the light water reactor and "the rapid loss of water used to cool the reactor could result in a serious safety problem," analyst Nick Hansen wrote on the institute's blog, 38 North.

North Korea has more experience with its restarted plutonium production reactor at Yongbyon but its "lack of airtight containment could lead to the escape of some radioactivity even in small accidents."

The published analysis comes after South Korean President Park Geun-Hye warned that Yongbyon could witness a Chernobyl-style disaster, one of a series of comments that enraged North Korea, whose official media accused her of speaking "nonsense gibberish."

The 38 North analysis downplayed the risks of a Chernobyl-scale disaster, saying Yongbyon was smaller than the Soviet-built station in Ukraine where a 1986 accident killed 30 people in an explosion and another 2,500 afterward in related illnesses.

"However, a radioactive release into the atmosphere or river would cause an expanded local area of contamination," the analysis said.

"Also, Pyongyang's likely lack of transparency could create a regional crisis, panicking the public in surrounding countries and raising tensions with governments anxious for further information."

North Korea knocked down a vital cooling tower in 2008 as part of a US-backed six-nation disarmament agreement. It has more recently vowed to boost its nuclear "deterrent" and conduct a "new" type of test in response to what the regime describes as US hostility.

- Concern on rights -

The US pointman on North Korea, Glyn Davies, held the talks with his counterparts Junichi Ihara of Japan and Hwang Joon-Kook of South Korea.

The three also pledged to focus on the "deplorable" human rights situation in North Korea after a UN commission said that Kim's regime was carrying out violations unprecedented in the modern world.

The three-way talks mark the latest return to diplomacy between South Korea and Japan, whose own relations are tense due to disputes related to wartime history.

US President Barack Obama recently held a breakthrough three-way meeting with Park and Japanese Prime Minister Shinzo Abe on the sidelines of a summit in The Netherlands.