The envoy's two-day visit is expected to focus in part on laying the groundwork for a future trip to the country by President Vladimir Putin, a Kremlin spokesman told the TASS news agency earlier.

It comes a day after Moscow dismissed US allegations that North Korea had begun supplying Russia with arms for its war in Ukraine.

North Korean leader Kim Jong Un last month travelled to Russia and invited Putin to visit his country.

Kim's summit with Putin fanned Western fears that Pyongyang could provide Moscow with weapons for its drawn-out war in Ukraine.

On Friday, the United States said arms shipments were already under way, with North Korea delivering more than 1,000 containers of military equipment and munitions to Russia in recent weeks.

Pyongyang was seeking a range of military assistance in return, including advanced technologies, White House national security spokesman John Kirby told reporters.

According to a graphic provided by the White House, a load of containers was shipped by sea from Najin in North Korea, to Dunay in Russia, between September 1 and October 1.

They were then delivered by rail to an ammunition depot about 290 kilometres (180 miles) from the Ukrainian border.

But the Kremlin on Tuesday said the United States had no evidence that weapons were being shipped.

"They report this all the time, without providing any proof," Kremlin spokesman Dmitry Peskov told Russian news agencies when asked about the reported arms shipments.

Washington-based analysts Beyond Parallel last week separately released satellite images showing what they termed an "unprecedented" build-up of train traffic along Russia's border with North Korea.

The flurry of activity "likely indicates North Korea's supply of arms and munitions to Russia", the group said in its report.

While Russia has ramped up production of shells this year to a forecast 2.5 million, analysts have suggested that could fall short of its needs on the battlefield.

Moscow's forces are firing about 60,000 rounds per day, according to Ukrainian figures.

North Korea is a mass producer of conventional weaponry and known to be sitting on large stocks of Soviet-era war material -- albeit in unknown condition.

- Top priority -

During his trip through Russia's far east last month, Kim said he would make North Korea's bilateral ties with Moscow the country's "number one priority".

Russia and North Korea are both under a raft of international sanctions -- Moscow for its invasion of Ukraine, and Pyongyang for its testing of nuclear weapons.

The tightening of their alliance comes as relations between the two Koreas are at a historic low, with the North conducting a record-breaking series of weapons tests this year and recently enshrining its status as a nuclear state in its constitution.

South Korea has in turn moved to tighten its security arrangements with traditional ally the United States while entering a new trilateral arrangement that also includes Japan.

Both US President Joe Biden and South Korean counterpart Yoon Suk Yeol have declared that any use of nuclear weapons by the North would mean "the end" of the regime in Pyongyang.

A US B-52 bomber capable of carrying nuclear weapons landed Tuesday at Cheongju Airport, about 100 kilometres south of Seoul, Yonhap news agency reported.

Its arrival came less than a week after the aircraft carrier USS Ronald Reagan docked in the southern port city of Busan, provoking anger from Pyongyang.

While B-52s have previously taken part in joint exercises over the peninsula, it marked the first time one has landed in the country since at least 2000, when such record-keeping began.

The bomber did a flyover at South Korea's massive ADEX defence exhibition before setting down in Cheongju.

South Korean media on Wednesday reported the bomber was expected to take part in a weekend aerial exercise involving planes from South Korea, Japan and the United States.