Earlier, the White House left open the possibility Obama could meet with Iranian President Hassan Rouhani. Both are scheduled to address the U.N. General Assembly Tuesday.
On the terror front, at least 161 people near a northeastern Nigeria town are believed dead from a terrorist attack after 87 more bodies were found Friday, officials said.
Fifty-five other bodies were found Wednesday along a road in Borno State after an attack by suspected Boko Haram gunmen last week, Abuja's Daily Trust reported.
In Kenya, fires set by gunmen inside Nairobi's Westgate Mall were brought under control Monday as Kenyan authorities secured the building where at least 62 people died in a terror attack during the weekend.
Joined by other officials, Obama and Nigerian President Goodluck Jonathan spoke to reporters Monday about the relationship between the two countries.
Obama said the relationship included helping Nigeria improve its rural electrification and making sure Nigerian elections in 2015 are democratic.
Obama said the United States wants to help Nigeria deal with threats in the north of the country in a way that doesn't violate human rights. Obama cited Boko Haram, "one of the most vicious terrorist organizations in the world."
On the al-Shabaab terror attack in Nairobi, Obama said: "We stand with them against this terrible outrage that's occurred, we will provide them with whatever law enforcement help that is necessary. ... 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."
Both presidents offered condolences to Kenya.
Jonathan said he appreciated help from the United States to help stabilize his country and the continent. Nigeria is Africa's second largest economy and Africa's largest trading partner with the United States.
"For you to fix the world , you must fix Africa. For you to fix Africa, you must fix Nigeria," Jonathan told Obama.
White House deputy national security adviser Ben Rhodes told reporters ahead of Obama's scheduled arrival in New York no meeting had been scheduled with Rouhani, whose arrival time was not immediately clear.
"The fact of the matter is, we don't have a meeting scheduled with President Rouhani," Rhodes told reporters in a conference call. "But again, we're always open to diplomacy if we believe it can advance our objectives. And in this instance, our objective is an Iran that meets its international obligations."
Those obligations include demonstrating its nuclear program is exclusively for peaceful purposes, he said.
Washington contends Tehran is well along toward developing an atomic weapons program.
Obama and Rouhani are to address the U.N. General Assembly's annual meeting within hours of each other Tuesday. This has raised speculation of the first face-to-face exchange between a U.S. and Iranian leader since President Jimmy Carter met with Shah Mohammad Reza Pahlavi in December 1977, 14 months before Iran's Islamic Revolution.
Rhodes said Obama has expressed his willingness since his 2007 pre-election campaign to have meetings directly with Iran with no preconditions if he saw an opportunity for progress.
Rouhani has said he is ready to meet with no preconditions.