Russian President Vladimir Putin

Russian President Vladimir Putin on Tuesday hailed North Korea for "firmly supporting" Moscow's war in Ukraine ahead of a visit to Pyongyang set to boost defence ties between the two nuclear-armed countries.

Putin is scheduled to touch down on Tuesday night for his first trip to the isolated nation in 24 years, with a confrontation between North and South Korean troops on their shared border highlighting regional security tensions.

Moscow and Pyongyang have been allies since North Korea's founding after World War II, and they have drawn even closer since Russia's invasion of Ukraine in 2022 led to the West isolating Putin internationally.

The United States and its allies have accused North Korea of supplying Russia with much-needed arms, including ballistic missiles to use in Ukraine.

The North has denied giving Russia military hardware but, ahead of his trip, Putin thanked Kim Jong Un's government for helping the war effort.

"We highly appreciate that the DPRK (North Korea) is firmly supporting the special military operations of Russia being conducted in Ukraine," Putin wrote in an article published by Pyongyang's state media on Tuesday.

Russia and the North are "now actively developing the many-sided partnership," Putin wrote.

The trip will elevate ties to a "higher level" the Russian leader wrote, adding it would help develop "equal cooperation" between the two allies.

Both countries are under rafts on UN sanctions -- Pyongyang since 2006 over banned nuclear and ballistic missile programs, and Moscow over the invasion of Ukraine.

Putin praised the North for "defending their interests very effectively despite the US economic pressure, provocation, blackmail and military threats that have lasted for decades."

He also hailed Moscow and Pyongyang for "maintaining the common line and stand at the UN."

North Korea has described allegations of supplying weapons to Russia as "absurd".

However it did thank Russia for using its UN veto in March to effectively end monitoring of sanctions violations, just as UN experts were starting to probe alleged arms transfers.

The United States voiced "concern" on Monday about the trip because of the security implications for South Korea as well as Ukraine.

The two Koreas have remained technically at war since their 1950-53, and the border dividing them is one of the most heavily fortified in the world.

"We know North Korean ballistic missiles are still being used to hit Ukrainian targets (and) there could be some reciprocity here that could affect security on the Korean peninsula," National Security Council spokesman John Kirby told reporter.

Highlighting those security concerns, South Korea said its troops fired at soldiers from the North who briefly crossed the border on Tuesday then retreated.

The South's military said it believed the North Korean soldiers accidentally crossed as they were fortifying the border, but said some of them were injured after detonating landmines.

NATO's Secretary General Jens Stoltenberg said Putin's trip showed how he was "dependent" on authoritarian leaders.

"Their closest friends and the biggest supporters of the Russian war effort -- war of aggression -- (are) North Korea, Iran and China," Stoltenberg said.

Ukraine's Foreign Minister Dmytro Kuleba urged the international community to counter "the lonely bromance" between Putin and Kim by increasing arms supplies to Kyiv.

"The best way to respond to it is to continue strengthening the diplomatic coalition for just and lasting peace in Ukraine and delivering more Patriots and ammunition to Ukraine," Kuleba told AFP.

North Korea is eager for high-end military technology to advance its nuclear, missile, satellite and nuclear-powered submarine programs, according to experts.

Kremlin aide Yuri Ushakov said the two leaders would sign possibly sign a "comprehensive strategic partnership treaty" to outline cooperation on "security issues", state-run Russian news agencies reported.

The North could promise "to provide Russia with continuing supplies of artillery, guided rockets for multiple rocket launchers, and short-range missiles to support Russia's operations in Ukraine," Bruce Bennett, senior defence analyst at RAND Corp. told Yonhap.

In return, it will want "Russia to provide a variety of advanced technologies," he said, plus "a substantial flow of Russian oil and food products along with hard currency payments."

Russian state media also flagged that Putin could seek to offer the North further support on beating US-led sanctions.