The United States has threatened to impose sanctions on Somalis who stand in the way of a UN-supervised roadmap for peace in Somalia.Assistant Secretary of State for African Affairs Johnnie Carson was speaking after making the first visit by a top US official in two decades.He said his trip to Mogadishu was to acknowledge "significant progress" made against al-Shabab militants.However, the al-Qaeda group still controls much of the country.The BBC's Gabriel Gatehouse, in Mogadishu, says the US envoy was in and out of the Somali capital in a matter of hours on Sunday, but during his key visit met the Somali president, prime minister and other members of the Transitional Federal Government.Mr Carson said the US would impose sanctions on anyone standing in the way of the political process now under way."The kind of action we must take against spoilers range from visa sanctions to travel sanctions to asset freezes," Mr Carson said."There is a level of vulnerability for all of those who might be spoilers."Mr Carson said the US would consider stationing officials in the capital permanently "when we believe that it is both appropriate and safe".He also dismissed as "absurd" al-Shabab's offer of a reward of 10 camels for information about US President Barack Obama's hideout.The mock offer came after the US put bounties of $3-7m (£2-4.5m) for various militant commanders last week. According to the UN-backed roadmap, a new parliament must be appointed and a new constitution adopted before 20 August.Our correspondent says the process is fraught with potential pitfalls - not least a number of former warlords who have financial and political interests in maintaining instability.But he says Somalia is now closer to establishing some sort of meaningful central government that it has been for more than 20 years.Mr Carson praised the African Union (AU) troops who helped push al-Shabab out of Mogadishu last August.Last month, AU troops also took the al-Shabab stronghold of Afgoye, on the outskirts of Mogadishu, while Kenyan troops are pressuring al-Shabab from the south and Ethiopian troops are deployed from the west.UN Special Representative for Somalia Augustine Mahiga told the BBC the strategy was an "innovative, functioning and promising partnership between the UN and a regional organisation - the African Union".