The African Union force "simply can not be expected to carry the burden of Somalia's security forever", the British prime minister said, noting that al-Shabab has tripled its attacks in the capital, Mogadishu.
Mattis told reporters flying with him back to Washington, that he had a private meeting with Somali President Mohamed Abdullah Mohamed, "and I came away from that heartened".
"After more than 20 years of a dramatic conflict that caused vast suffering to the Somali people... all the conditions are now met for Somalia to be a success story", said United Nations Secretary-General Antonio Guterres.
British Defence Secretary Michael Fallon, (2L), President of Somalia, Mohamed Abdullahi Mohamed (C) and British Foreign Secretary Boris Johnson (2R) attend a national security session at the London Somalia Conference at Lancaster House in London on Thursday.
Aid groups report that a third starvation warning is "on its way" to Somalia - a crucial turning point for response, they say - as governments from across East Africa and key partner countries meet for the Somalia Pledging Conference in London, United Kingdom, on 11 May.
"In this year alone the European Union and its member states will have invested over €800 million in Somalia, which will bring our total support to €3,5 billion for the 2015-2020 period", Mogherini said.
May made a long-term commitment to reinforce Somali security forces and insure that assistance is rapidly provided to the country.
UK Prime Minster Theresa May is also expected to speak at the meeting.
Moon to Xi: Let's Talk
Moon has said the decision had been made hastily and his government should have the final say on whether to deploy it. He has been cautious about the recent decision to deploy a controversial USA anti-missile system in South Korea.
A legitimate Federal Government was established in 2012 in Somalia, and Al-Shabaab was pushed out of several key cities by the UN Security Council-mandated African Union Mission to Somalia (AMISOM).
USA forces also assist Somali troops in operations against Shabaab, and last week a Navy SEAL was killed during a night-time raid outside Mogadishu.
"This morning, we presented the revised humanitarian response plan seeking an additional $900 million the end of the year", said Guterres on the sidelines of an worldwide conference on Somalia organized by U.K's Prime Minister, Theresa May.
Somalia has frequently topped the Fragile State Index due to threats from Islamic extremism, piracy and starvation.
Mr. Guterres noted that Somalia is at a moment which is both "tragic and hopeful" and "can be in the near future the kind of good news that we badly need" in the world.
Somalia has signed a security pact with the worldwide community aimed at building a national force to fight al-Shabab militants.
"The longstanding arms embargo on Somalia severely restricts our ability to procure heavy weapons", Mohamed said.
No fewer than 680,000 people have been displaced due to drought since November 2016, including 7,000 people who have crossed into neighboring Ethiopia and Kenya.
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