Russian President Vladimir Putin is waging a secret campaign in Europe involving assassination plots, sabotage and arson to disrupt public support for Ukraine. A senior NATO official, who spoke to reporters on condition of anonymity, said this on Tuesday, Voice of America reported. In addition, he said, Russia lacks enough ammunition and troops to launch a major offensive in Ukraine and needs to secure significant supplies of ammunition from countries other than those it already has. "Today we see still very heavy Russian losses. Russia is trying to take over [Ukrainian] territories. We see a significant improvement in Ukrainian defense," the NATO official said. He suggested that Russia would be able to maintain its military economy for another three to four years.The Ukrainian Armed Forces have been on the defensive on the battlefield for months as Russian forces continue to exert heavy offensive pressure and make slow progress in eastern Ukraine. A NATO official said it would take some time for Ukraine to accumulate the necessary ammunition and personnel for new large-scale offensive operations. "We see things improving day by day," he added. NATO has responded to a secret Russian campaign to disrupt support for Ukraine in Europe by issuing two statements warning Moscow that the alliance is aware of its actions, a NATO spokesman said.