Vladimir Putin’s suggestion that Russian could supply weapons to North Korea is “incredibly concerning”, a senior US official has said, days after Putin and the North’s leader, Kim Jong-un, signed a defence pact that requires their countries to provide immediate military assistance if either is attacked. Matthew Miller, a US state department spokesperson, said the provision of Russian weapons to Pyongyang “would destabilise the Korean peninsula, of course, and potentially … depending on the type of weapons they provide … violate UN security council resolutions that Russia itself has supported”. The US secretary of state, Antony Blinken, and South Korea’s foreign ministry said the treaty between Russia and North Korea posed a “serious threat” to peace and stability on the Korean peninsula. Blinken said the US would consider “various measures” in response to the pact, which elevated ties between the sanctions-hit states to their highest level since the cold war. Friction over shipments of weapons to both sides in the war in Ukraine has worsened this week, amid speculation that Putin and Kim discussed additional supplies of North Korea missiles and ammunition for use by Russian forces when they met in Pyongyang on Wednesday. During a state visit to Vietnam on Thursday, Putin said reciprocal supplies of Russian weapons to the North would be an appropriate response to the west’s supply of weapons to Ukrainian forces. “Those who send these [missiles to Ukraine] think that they are not fighting us, but I said, including in Pyongyang, that we then reserve the right to supply weapons to other regions of the world, with regard to our agreements” with North Korea, Putin said. “I do not rule this out.” Putin, who met Kim for the second time in nine months, also warned South Korea that it would be making a “big mistake” if it decided to supply arms to Ukraine. “I hope it doesn’t happen,” he told reporters in Hanoi. “If it happens, then we will be making relevant decisions that are unlikely to please the current leadership of South Korea.” South Korea, a growing arms exporter with a well-equipped military backed by the United States, has provided non-lethal aid and other support to Ukraine, and joined US-led sanctions against Moscow. But it has a longstanding policy of not supplying weapons to countries that are at war. But on Friday, Chang Ho-jin, the national security adviser to the South Korean president, Yoon Su Yeol, said Seoul would reconsider its stance on providing arms to Ukraine. The head of Nato, Jens Stoltenberg, also voiced concern that Russia could help the North further its ballistic and nuclear missile programmes – both of which have made significant progress despite years of UN security council sanctions. US officials believe North Korea wants to acquire fighter aircraft, surface-to-air missiles, armoured vehicles, materials and equipment to build for ballistic missile production, and other advanced technologies from Moscow. The US and South Korea say there is evidence that Pyongyang has already provided significant numbers of ballistic missiles and artillery shells to Russia. The North has described the allegations as “absurd”. Exports of Russian weapons to the North would add to tensions on the Korean peninsula and, according to some experts, risk intensifying a regional arms race that has drawn in South Korea and Japan – both US allies. Relations between the two Koreas have deteriorated dramatically in recent weeks with the resumption of cold war-era psychological warfare that included North Korea using balloons to drop huge quantities of rubbish on the southern side of the countries’ border. Seoul has responded by broadcasting anti-North Korean propaganda via loudspeakers. On Friday its troops fired warning shots after soldiers from the North reportedly crossed the border for the third time this month. There is also evidence that North Korea is building walls at points along the border, days after several of its soldiers were reportedly killed or injured while clearing land in areas packed with mines. The BBC said high-resolution satellite imagery of a 7km stretch of the border appeared to show at least three sections where barriers have been built. “My personal assessment is that this is the first time they’ve ever built a barrier in the sense of separating places from each other,” it quoted Dr Uk Yang, a military and defence expert at the Asan Institute for Policy Studies in Seoul, as saying. Experts believe the intrusions could be related to the large number of troops the North Korea has deployed to fortify the border, possibly to prevent civilians and soldiers from defecting to the South. Putin’s visit to Vietnam, where he was given a 21-gun salute on Thursday, has also caused unease in Washington. In response, the US’s most senior diplomat for east Asia, Daniel Kritenbrink, will visit Vietnam on Friday and Saturday to underline Washington’s commitment to working with Hanoi to ensure a “free and open” Indo-Pacific region.