At least 13 people were killed and five injured on Tuesday in twin explosions near the airport in Mogadishu, Somalia security officials said.Most of the casualties were UN contractors who operated near the explosion sites, Xinhua news agency quoted a police official as saying.The first explosions hit the entrance of the airport while the other one hit a checkpoint nearby, another official said.There were huge blasts near the airport, a witness said. "I heard two huge explosions and there were bodies of people strewn over the place."Security forces have cordoned off the area.Al-Shabaab militant group has claimed responsibility for the attack.