By Adrian Croft BRUSSELS (Reuters) - NATO sees no sign that Russia is withdrawing troops from the Ukraine border and will look at all options to boost the alliance's defenses, its secretary-general said on Tuesday. Ministers from the 28 alliance members are meeting in Brussels for the first time since Russia's military occupation and annexation of Ukraine's Crimea region triggered the worst East-West crisis since the Cold War. They will discuss ways to boost NATO's military presence in former communist central and eastern Europe to reassure allies rattled by Russia's moves. "Unfortunately, I cannot confirm that Russia is withdrawing its troops.