‘Russia is the biggest threat to US national security and America must boost its military presence throughout Europe even as Nato allies face budget challenges and scale back spending’ said Deborah James, US Air Force Secretary, on Wednesday after a series of visits and meetings with American allies across Europe, including Poland. She said Washington was responding to Russia's recent ‘worrisome’ actions by boosting its presence across Europe and would continue rotational assignments of F-16 fighter squadrons. ‘This is no time to signal a lack of resolve in the face of these Russian actions.’ she said. Given the tensions, the air force is continuing its effort to reduce US reliance on Russian RD-180 rocket engines for military and intelligence satellite launches. She said the air force was also working hard to defend its weapons systems and networks against growing cyber-attacks.