North Korea has seized upon Russia’s international isolation following President Vladimir Putin’s invasion of Ukraine to foster closer ties, threatening international efforts to pressure Pyongyang over its illicit nuclear weapons programme. US officials said on Tuesday that Russia has purchased millions of rockets and artillery shells from North Korea as western sanctions begin to choke Moscow’s supply of weapons. The disclosure is the latest sign of warming ties between the countries, after Pyongyang broke with Chinese policy in July and recognised the breakaway territories of Luhansk and Donetsk in Russian-occupied Ukraine.