UN chief Ban Ki-moon has laid a wreath in conflict-hit Nigeria's capital to commemorate the fourth anniversary of a deadly attack on the global body by Boko Haram militants.
The secretary-general hugged several relatives of the victims of the bombing during a ceremony at United Nations House and praised "the extraordinary fortitude and determination" of the survivors.
"Our fallen colleagues and partners will be remembered this morning with a moment of silence in many places," he said.
"But nowhere are the memories of these colleagues more immediate, more vivid and more compelling than here in Abuja. We will remember them forever as truly the best of humanity."
At least 21 people were said to have died when a bomb-laden car exploded at UN House, the headquarters for about 400 employees, on August 26, 2011.
The death toll rose to 24 in the months that followed, according to the United Nations, with 12 non-UN staff among the victims.
The radical Islamist sect Boko Haram, blamed for the deaths of at least 15,000 people during a six-year-insurgency, mainly in the country's northeast, claimed responsibility for the attack.
"The terrorists attacked the United Nations and destroyed the lives of many colleagues. But we have a mandate to build. To better the lives of people in need," Ban said.
Ban arrived in Abuja on Sunday for a two-day visit during which he is due to hold talks with Nigeria's President Muhammadu Buhari, with violent extremism high on the agenda, as well as development and climate change.
"Across our world, we see insecurity, inequality, growing divides," Ban told a private meeting of state governors in Abuja late on Sunday, according to a UN statement.
"Here in Nigeria, you know the challenges all too well - including the rise of extremism and the lack of equal opportunity."