North Korea is working to ensure its nuclear and ballistic missile capabilities cannot be destroyed by military strikes, UN monitors have said.

The report comes ahead of a meeting between US and North Korean officials to prepare a second denuclearisation summit.

The US special envoy for North Korea, Stephen Biegun, will meet his North Korean counterpart on Wednesday in Pyongyang to prepare for a summit later this month between Donald Trump and North Korean leader Kim Jong-un, the US State Department said on Monday.

Mr Biegun has said he hoped the meeting with newcounterpart Kim Hyok Chol would map out "a set of concrete deliverables" for the summit between Mr Trump and Mr Kim.

Mr Biegun, who held talks with South Korean officials in Seoul on Sunday and Monday, said he would be aiming for "a roadmap of negotiations and declarations going forward, and a shared understanding of the desired outcomes of our joint efforts".

Singapore Summit: historic first meeting between Trump and Kim Jong-un Show all 20 1 /20 Singapore Summit: historic first meeting between Trump and Kim Jong-un Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump and Mr Kim leave following their historic meeting AFP/Getty Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump and Mr Kim sign a document committing to peace between their nations, amongst other things EPA Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump holds up a document that both he and Mr Kim have signed AFP/Getty Singapore Summit: historic first meeting between Trump and Kim Jong-un The two leaders reach out to shake hands for the first time AP Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump and Mr Kim shake hands as they meet for the first time Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump and Mr Kim walk off stage, following their historic meeting Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump and Mr Kim take a stroll after their meeting on June 12 Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump and Mr Kim interrupt their stroll to talk to the media AP Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Kim and Mr Trump look out over a balcony at the Capella hotel, where they held their meeting on June 12 Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump takes questions at the press conference following his meeting with with Mr Kim AP Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump appreciates the crowd in the press conference following his meeting with Mr Kim Getty Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump travels to the Istana ahead of the summit with Kim Jong Un on June 12 Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Kim Jong Un travels to the Istana ahead of the summit with President Trump on June 12 Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Kim poses for a photo with Singapore's Finance Minister Vivian Balkrishnan and Education Minister Ong Ye Kung ahead of the summit with Mr Trump Twitter/Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump shakes hands with Singapore Prime Minister Lee Hsien Loong, meeting on Monday June 11 in the Istana, Singapore's presidential palace AP Singapore Summit: historic first meeting between Trump and Kim Jong-un At a working lunch on Monday, Singapore's Prime Minister surprised Mr Trump with a birthday cake, in early celebration of his 72nd birthday later in the week EPA Singapore Summit: historic first meeting between Trump and Kim Jong-un US Secretary of State Mike Pompeo answers questions about the summit at a press briefing in Singapore on Monday, June 11 Getty Singapore Summit: historic first meeting between Trump and Kim Jong-un US President Donald Trump arrives at the Paya Lebar Air Base in Singapore on June 10, two days prior to his summit with North Korean leader Kim Jong Un Reuters Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump and Mr Kim met on Tuesday June 12 in the Capella hotel on the island resort of Sentosa, just off the south coast of Singapore AFP/Getty Singapore Summit: historic first meeting between Trump and Kim Jong-un Mr Trump boards Air Force One following the summit AFP/Getty

South Korean officials said they and the US could be looking at a compromise that would see the dismantling of the North's main Yongbyon nuclear complex, which could be reciprocated by US measures including formally ending the 1950-53 Korean War and setting up a liaison office.

But United Nations (UN) sanctions monitors said in a confidential report, submitted to a 15-member UN Security Council sanctions committee and seen by Reuters on Monday, that they had "found evidence of a consistent trend on the part of [North Korea] to disperse its assembly, storage and testing locations".

The North Korean mission to the UN did not immediately respond to a request for comment on the report, which was submitted to Security Council members on Friday.

The first summit between Mr Trump and Mr Kim last June in Singapore yielded a vague commitment by Mr Kim to work towards the denuclearisation of the Korean peninsula, where US troops have been stationed since the 1950-53 Korean War.

The Vietnamese resort town of Danang is seen as the most likely location for the next summit.

Last Thursday, Mr Trump hailed "tremendous progress" in his dealings with North Korea, but the view in the US is that it has yet to take concrete steps to give up its nuclear weapons programme.

North Korea has complained the US has done little to reciprocate its freezing of nuclear and missile testing and dismantling of some nuclear facilities.

North Korea destroys 10 guard posts to lower tensions

It has also repeatedly urged a lifting of punishing US-led sanctions, a formal end to the war, and security guarantees.

The UN report said sanctions were proving ineffective.

"The country continues to defy Security Council resolutions through a massive increase in illegal ship-to-ship transfers of petroleum products and coal," the sanctions monitors found.

"These violations render the latest UN sanctions ineffective."

The monitors said they had evidence of one unprecedented prohibited petroleum product transfer of more than 57,600 barrels, worth more than $5.7 million (£4.4 million).

North Korea has said it will never unilaterally give up its nuclear weapons unless the US first removes any threat to it. North Korea has long demanded US troops be withdrawn as a condition for peace.

The Korean War ended with an armistice that left the two Koreas technically still at war.

The US has stressed its troops are not a bargaining chip and South Korea has said US troops in the South were unrelated to any future peace treaty and American forces should stay even if such an agreement is signed.

The US State Department said on Monday Washington and Seoul had reached an agreement "in principle" on sharing the cost of stationing US troops in the Asian country.

CNN quoted a State Department official as saying that under the revised agreement, South Korea would boost its contribution to nearly $1bn (£770m).

A 2014 deal that expired last year required South Korea to pay about 960 billion won (£660 million) a year for keeping some 28,500 US troops in South Korea. The allies had appeared unable to strike an accord to renew the deal despite 10 rounds of talks since March.