THE highest ranking U.S. official to visit Somalia’s capital in years landed in Mogadishu on Sunday in another sign of improving security in the Horn of Africa’s most chaotic nation.Johnnie Carson, the assistant secretary of state for African affairs, arrived at the seaside airport Sunday morning, said Lt. Col. Paddy Ankunda, the spokesman for the African Union military force in Somalia.A U.S. Embassy spokesman in neighboring Kenya said he could not comment. However, Carson is scheduled to speak at a news conference in Nairobi — Kenya’s capital — later Sunday.African Union troops pushed al-Shabab fighters out of the capital in August, allowing markets and even the arts to flourish for the first time in years. The U.N.’s top official visited in December — the first visit by the U.N. secretary-general in nearly two decades.The U.S. does not have an embassy in Somalia, though embassy officials from neighboring Kenya have visited Somalia in recent months and years. The last senior U.S. government official to visit Mogadishu appears to have been a visit by Gen. Anthony Zinni in 1997.Al-Shabab, which only two years ago controlled most of Mogadishu, now faces military pressure on three sides. African Union troops last month attacked and took control of Afgoye, an al-Shabab stronghold on the outskirts of Mogadishu. Kenyan troops are moving toward the al-Shabab-controlled port city of Kismayo in the south, and Ethiopian troops are pressuring militant fighters in Somalia’s west.Somali government leaders have a lot of work to do over the next two months. The U.N. mandate that gives power to Somalia’s Transitional Federal Government expires Aug. 20. Somali elders are supposed to pass a new constitution and vote on a new, smaller parliament before then, and a vote to elect a new president and prime minister are also to be held on or before Aug. 20.The upcoming political transitions are likely the reason behind Carson’s visit. U.S., European and other world leaders are demanding progress from Somalia’s politicians to burnish the military progress being made by the African Union military force.