Famine in parts of southern Somalia has killed tens of thousands of people, mostly children, the United Nations said Wednesday in an official declaration of what aid officials describe as the worst humanitarian crisis in the troubled country in two decades.

The famine declaration comes months after U.N. and other aid agencies began sounding the alarm about a devastating drought in the Horn of Africa, where an estimated 10 million people need help. The crisis has been aggravated by civil strife, the lowest rainfall rates in half a century and sharp increases in food prices.

“Somalia is facing its worst food security crisis in the last 20 years,” Mark Bowden, the top U.N. official in charge of humanitarian aid to the country, told reporters, adding that $300 million is needed within two months to help alleviate the crisis.

Somalia has grappled with civil war and ineffective governments since 1991, when Mohamed Siad Barre’s regime was toppled. Today, a weak and corrupt transitional government, backed by the United States and its allies, is in place, with little ability to address the famine. Much of its energy is focused on preventing the capital, Mogadishu, from being overtaken by the al-Shabab militia, a group linked to al-Qaeda that seeks to turn the country into an Islamic emirate.

On Wednesday, the United States announced that it would give $28 million in aid to Somalis on top of the more than $431 million in food and other emergency assistance it has provided this year to the eastern Horn of Africa.

But some aid agencies accused the United States and other Western donors of failing to respond to the distress in Somalia quickly enough, despite numerous calls for assistance.

“The crisis has been building for several months, but the response from international donors and regional governments has been mostly slow, inadequate and complacent,” Fran Equiza, the regional director of the British aid agency Oxfam, said in a statement. “There has been a catastrophic breakdown of the world’s collective responsibility to act.”

Equiza said there was still an $800 million shortfall in funding, adding that “by the time the U.N. calls it a famine, it is already a signal of large-scale loss of life.”

In 1992, hundreds of thousands of Somalis starved to death, prompting a U.S.-led peacekeeping force to intervene. Within months, the force was engaged in an intense operation to uproot Somali warlords. It eventually withdrew after 18 American soldiers were killed in a battle the following year, an incident portrayed in the book and movie “Black Hawk Down.”

This time, the famine is unfolding in the southern Somali regions of Bakool and Lower Shabelle, which are largely controlled by al-Shabab. Bowden said that nearly half of Somalia’s 3.7 million people face hunger, malnutrition and other related problems. Of those, 2.8 million live in the south.

“If we don’t act now, famine will spread to all eight regions of southern Somalia within two months, due to poor harvests and infectious diseases,” Bowden said. “We still do not have all the resources for food, clean water, shelter and health services to save the lives of hundreds of thousands of Somalis.”

Famine, said Bowden, is officially declared when acute malnutrition rates among children exceed 30 percent and when more than two people per 10,000 die every day.

Today, malnutrition rates in Somalia are the highest in the world; in some parts of the south, more than half of all children are severely malnourished. In some areas of Bakool and Lower Shabelle, six children per 10,000 younger than 5 are dying every day, Bowden said.

“In Somalia, 20 years without a central government and the relentless terrorism by al-Shabab against its own people has turned an already severe situation into a dire one that is only expected to get worse,” Secretary of State Hillary Rodham Clinton said Wednesday.

Humanitarian funding to help Somalia has declined since 2008, U.N. officials say. The United States, once Somalia’s largest donor, has reduced humanitarian funding by 88 percent, according to a September 2010 report by the U.N. Office for the Coordination of Humanitarian Affairs. In 2008, the United States provided $237.4 million. In 2009, it gave $99.6 million; in 2010, roughly $28 million.

This year, the United States has given $78 million, including the $28 million announced Wednesday, U.S. officials said.

The decline in U.S. assistance to Somalia came after the U.S. Office of Foreign Assets Control, concerned about the diversion of aid by Islamist militants, imposed restrictions on agencies working in areas controlled by al-Shabab and froze some funding in March 2009.

U.N. officials are also concerned that Washington’s intensifying targeting of al-Shabab through drone strikes and other means could impede their access to Somalis in famine-stricken areas. “It does complicate our efforts,” Bowden said. “It increases their suspicions of humanitarian organizations.”

Since early 2010, al-Shabab has also prevented aid agencies from delivering assistance to large swaths of Somalia. The militants lifted the ban this month, but Bowden said the United Nations was still having a “dialogue” with them to allow international aid workers to enter famine-afflicted areas.

“Every day of delay in assistance is literally a matter of life or death for children and their families in the famine-affected areas,” Bowden said.

In her statement Wednesday announcing the new U.S. aid, Clinton said the United States remains “cautiously optimistic that al-Shabab will permit unimpeded international assistance in famine-struck areas.”