Somalia is at a critical point and must seize this moment to establish a stable government, US Secretary of State Hillary Clinton told a major conference on the Horn of Africa nation on Thursday.

She said the United States would push for sanctions, including travel bans and asset freezes, for those "standing in the way" of progress, and would also seek to keep up the pressure on the Shebab Islamist group.

But Clinton also promised support, pledging an extra $64 million in humanitarian assistance to the region to help improve the lives of ordinary Somalis, blighted by famine and civil war for the past 21 years.

"For decades, the world has focused on what we could prevent from happening in Somalia -- be it conflict, famine, or other disasters. Now we are focused on what we can build. The opportunity is real," she said.

Speaking at the start of the London conference gathering Somali leaders plus representatives from around 50 countries and organisations, Clinton said: "We have come together at a critical halfway point."

She said the international community would not extend the mandate of the transitional government (TFG) beyond August, saying: "It is past time for that transition to occur, and for Somalia to have a stable government."

She acknowledged that the work required before that date, including convening an assembly to approve a constitution, form a new parliament and elect a president and speaker, were "ambitious".

"But the people of Somalia have waited for many years. They have heard many promises, and have seen many deadlines come and go," she said.

"It's time to buckle down and do the work that will bring stability to Somalia for the first time in many of its people's lives."

She said anyone trying to obstruct progress "will not be tolerated".

"We will encourage the international community to impose further sanctions -- including travel bans and asset freezes -- on people inside and outside the TFG who seek to undermine Somalia's peace and security," she said.

"If you are standing in the way, you should be held accountable."

Clinton said the Shebab movement was "weakening" but said the international community "must keep up the pressure", welcoming the United Nations' decision on Wednesday to boost peacekeeping forces as "a good step".

She also addressed the problem of pirates who attacked international ships off the coast of Somalia, urging more countries to offer to prosecute and jail those arrested in the absence of a judicial system in Somalia itself.