Multiple North Korean soldiers 'killed or injured' by exploding landmines inside Demilitarized Zone just hours before Putin's meeting with Kim Jong Un

Multiple North Korean soldiers have been killed or injured in a landmine explosion inside the Demilitarized Zone as South Korea fired warning shots early Tuesday morning.

The South Korean military said it fired the shots to repel North Korean soldiers  who temporarily crossed the rivals' land border for the second time this month.

They said an estimated 20 to 30 North Korean soldiers, while engaging in unspecified construction work on the northern side of the border, crossed the military demarcation line that bisects the countries at around 8.30am local time.

The North Korean soldiers then retreated after the South broadcasted warnings and fired shots, South Korea's Joint Chiefs of Staff said, adding that they did not see any suspicious activity after. 

As they fled, an unspecified number of North Korean soldiers were injured in a landmine explosion, the Yonhap News Agency reports, according to the AFP.

'North Korean military suffers multiple casualties due to a mine explosion during work,' the report said, without giving further details.

North Korean soldiers were reportedly 'killed or injured' by exploding landmines inside the Demilitarized Zone Tuesday morning

The South Korean military said it fired warning shots to repel North Korean soldiers who temporarily crossed the rivals' land border for the second time this month 

The South previously fired warning shots on June 11 after another group of North Korean soldiers briefly crossed the MDL. 

But Tuesday's incident comes just hours before Russian President Vladimir Putin is expected to visit the communist country for a two-day visit - his first in 24 years. 

South Korea's Joint Chiefs of Staff said it occurred in a different area along the central frontline region. 

They said they don't believe the North Korean soldiers intruded the border intentionally and that the North did not return fire.

The Joint Chiefs noted that the North has been increasing construction activity in frontline border areas, such as installing suspected anti-tank barriers, reinforcing roads and planting land mines.

In recent weeks, South Korean officials said they also observed several explosions suspected to have been caused by mines in areas where North Korean soldiers were deployed for construction work, but that the activities continued despite an unspecified number of injuries or deaths.

The North has been increasing construction activity in frontline border areas, such as installing suspected anti-tank barriers, reinforcing roads and planting land mines

South Korea's Joint Chief of Staff believe the North will continue to expand its border construction activities 

The Joint Chiefs of Staff now anticipates North Korea will continue to expand its border construction activities, which it said could be aimed at making it harder for North Korean civilians or soldiers to escape to the South as Pyongyang´s leadership attempts to strengthen its control over its people.

Meanwhile, Putin is expected to arrive in Pyongyang late Tuesday with an entourage of government members and advisers, including those responsible for Russian military and weapons procurement, The Guardian reports. 

Over the past few years, the two countries have deepened their ties, with North Korea supplying Russia with millions of rounds of Soviet-era munitions, as well as ballistic missiles and electronic surveillance equipment, in its fight against Ukraine.

US Secretary of Defense Lloyd Austin told lawmakers last month that the North Korean supplies - in addition to drones supplied from Iran - have helped the Russian military 'get back on their feet.

In return, Russia is believed to have provided aid to North Korea's satellite program, as well as other arms, economic aid and diplomatic support.

The incident on Tuesday comes just hours before Russian President Vladimir Putin is expected to visit the communist country 

Over the past few years, the two countries have deepened their ties

In an op-ed for North Korean state military ahead of the visit, Putin said he highly appreciates North Korea's firm support of its invasion of Ukraine.

He vowed that the two countries would continue to 'resolutely oppose' what he described as Western ambitions 'to hinder the establishment of a multi-polarized world order based on mutual respect for justice.'

Putin also said Russia and North Korea will develop trade and payment systems 'that are not controlled by the West' and jointly oppose sanctions against the countries, which he described as 'unilateral and illegal restrictive measures.'

North Korea is under heavy U.N. Security Council economic sanctions over its nuclear weapons and missile programs, while Russia is also grappling with sanctions by the United States and its Western partners over its aggression in Ukraine.

Russian agencies also said Monday that the two leaders will sign 'important documents,' The Guardian reports.

Those may include a 'comprehensive strategic partnership treaty' which will outline future cooperation and deal with 'security issues,' Kremlin aide Yuri Ushakov was quoted as saying in Russian state media. 

In the meantime, the White House remains apprehensive about the two countries forming closer bonds.

'We're not concerned about the trip,' National Security Council Spokesman John Kirby told reporters.

'What we are concerned about is the deepening relations between these two countries.'