"The United States welcomes today's convening of Somalia's new federal parliament," White House spokesman Jay Carney said.
Local media said 211 of 275 lawmakers had so far been named. Carney said Washington urged "those remaining Somali communities that have not yet nominated their members of parliament to do so with urgency."
"We look forward to parliament expeditiously completing all remaining tasks," the White House press secretary said. The body has yet to choose a speaker, and then must elect a new president.
The US warned it would not accept any interference in the creation of a new government in Somalia, which has not had a stable central government since the 1991 ouster of dictator Mohamed Siad Barre, which sparked two decades of chaos.
"Any attempt to impede the political transition will not be tolerated," Carney said.
"All parties must work in a fair and transparent manner and will be held accountable for any failure to do so," he added, pledging US support in efforts "to achieve a better future for the people of Somalia."
The swearing-in, held on the tarmac of the airport in the capital Mogadishu, was the culmination of a UN-backed process in which lawmakers were chosen by a group of 135 traditional elders.
It brought an official end to Somalia's transitional government after eight years of political infighting and rampant corruption.