John Bolton, a former US national security adviser and ambassador to the United Nations, told Sky News the Russian president is in "greater trouble than at any point since the invasion". Vladimir Putin's mounting problems in Ukraine make Russia's use of a tactical nuclear weapon more likely, a senior US expert has told Sky News. Tactical nuclear weapons are designed to be used on a battlefield. John Bolton, a former US national security adviser and ambassador to the United Nations, said that the Russian president is in "greater trouble than at any point since the invasion". Ukraine makes biggest breakthrough in the south - live updates Discussing the Russian "annexation" of four occupied areas in eastern and southern Ukraine following a series of referendums described by Kyiv and the West as a "sham", Mr Bolton said: "Obviously he (Putin) thought it would be a morale booster for Russia, (that) it would help lay the groundwork for future borders with Ukraine. "But the fact is, at least