KYIV, Ukraine (AP) — Russian President Vladimir Putin is moving to tighten security along a key bridge to Crimea after an explosion caused part of the bridge to collapse. The Kerch bridge is an important supply artery for the Kremlin’s faltering war effort in southern Ukraine. The 12-mile-long bridge is also a symbol of Russia’s claim to control the territory, which it annexed from Ukraine in 2014. Nobody has claimed responsibility for the blast early Saturday. Russian authorities say it was caused by a truck bomb, which set some train tanker cars on fire. Train and automobile traffic on the bridge were suspended temporarily. Automobile service was restored later in the day on just part of the bridge.