World leaders have gathered in London for a one-day conference aimed at coming up with a coordinated response to instability in Somalia.

British Prime Minister David Cameron said at the outset of Thursday's meeting that the conference is not meant to impose international solutions on Somalia. But he said the world will “pay a price” if it fails to help the Horn of African nation recover from violence, famine and poverty.

U.S. Secretary of State Hillary Clinton told the conference that Somalia is at a “critical” point. She said the world must help Somalia establish a stable government and keep up the pressure on the al-Qaida-linked insurgent group al-Shabab.

Clinton said the international community would not extend the mandate of Somalia's weak transitional government beyond August, saying it is “past time” for Somalia to have permanent, representative leadership.

U.N. Secretary-General Ban Ki-moon also spoke at the opening of the conference, which is bringing together representatives of 40 countries.

Somalia has endured two decades of civil war and poverty since the fall of its last stable government in 1991. More recently, it has struggled to deal with a devastating famine, as well as pirates and al-Shabab, which is seen as a threat to regional security.

The militants control large portions of Somalia but have been losing ground to offensives by Ethiopian, Kenyan, and African Union troops.

Clinton said the U.S. supports all Somalis who denounce violence, but said Washington is “adamantly opposed” to negotiating with al-Shabab. She said Washington is working to impose sanctions against all who seek to undermine Somali security or who delay the political transition.

She also announced an additional $64 million in U.S. humanitarian assistance for the Horn of Africa.