US President Barack Obama speaks during his meeting with Nigeria's President Goodluck Jonathan in New York Sept 23, 2013. Obama on Monday called the attack in a Nairobi mall a "terrible outrage" and said the United States was providing all the cooperation it could to Kenya. [Photo/Agencies]

Obama immersed himself into diplomacy shortly after arriving from Washington, sitting down for talks with Nigerian President Goodluck Jonathan and presiding over a meeting of civil society experts who were critical of governments that crack down on non-governmental aid organizations.

Obama on Tuesday is to give his annual address to the UN General Assembly, a speech that this year will face greater scrutiny because of diplomatic efforts to contain Syria's chemical weapons and Iran's nuclear program.

A weekend attack at Nairobi's Westgate mall carried out by ?al Shabaab, a militant Somali Islamist group, killed at least 62 people, a sharp reminder that al-Qaida-type violence is not limited to the Middle East.

"The United States will continue to work with the entire continent of Africa and around the world to make sure that we are dismantling these networks of destruction," Obama said.

Obama, whose father was from Kenya, said he had spoken to Kenyan President Uhuru Kenyatta and pledged US support.

"We stand with them against this terrible outrage that's occurred. We will provide them with whatever law enforcement support that is necessary. And we are confident that Kenya will continue to be a pillar of stability in Eastern Africa," Obama said as he met the Nigerian president.

White House deputy national security adviser Ben Rhodes told reporters Obama would bring up the Kenya violence in his UN speech, which will also covers events in the Middle East and North Africa.

"The fact of the matter is al Shabaab is precisely the type of issue that we are increasingly confronted with. As al Qaeda's core is degraded in Afghanistan and Pakistan, we see affiliates take root in different parts of the world," Rhodes said.

Nigeria's Jonathan said during his meeting with Obama that he sympathized with Kenya. Nigeria has battled the Islamist insurgency group Boko Haram, which wants to establish a breakaway Islamic state. The Islamists are seen as the main security threat in Africa's top oil producer.

"Terror anywhere in the world is terror on all of us," said Jonathan.