By Alexandra Alper and Jeff Mason WASHINGTON (Reuters) -U.S. President Joe Biden and his European counterparts will announce new sanctions against Russia and new measures to tighten existing sanctions during his trip to Brussels this week, national security adviser Jake Sullivan said on Tuesday. Biden also will discuss longer-term adjustments to NATO force posture and contingencies in the case of nuclear weapons use, Sullivan said. Biden also will announce “joint action” on enhancing energy security in Europe, which is highly reliant on Russian gas. The United States and its allies have impose…