ABUJA - UN chief Ban Ki-moon laid a wreath in conflict-hit Nigeria's capital on Monday 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 around 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."

Ban touched down at Abuja's international airport just hours after the military revealed suspected Boko Haram fighters had ambushed a convoy carrying Nigeria's army chief-of-staff Lieutenant General Tukur Buratai.

The officer was unharmed but one soldier and 10 militants died in the ensuing firefight, in Faljari village, east of Borno State capital Maiduguri, according to army spokesman Sani Usman.

Boko Haram has stepped up its attacks in Borno and two neighbouring states in its northeastern heartland since Buhari came to power on May 29.

The Islamists have also carried out deadly ambushes across Nigeria's borders and in recent weeks suicide bombers, many of them women, have staged several attacks in Nigeria, Cameroon and Chad.

The fresh wave of violence has claimed more than 1,000 lives over the last three months, dealing a setback to a four-country offensive launched in February that had chalked up a number of victories against the jihadists.

An 8,700-strong Multi-National Joint Task Force, drawing in Nigeria, Niger, Chad, Cameroon and Benin, is expected to go into action soon.

Ban was due to meet Buhari later on Monday, before dining with captains of industry and discussing "democracy, human rights and countering violent extremism" in a meeting at the foreign ministry.

The secretary-general, on his second visit since taking office in 2007, last came to Abuja in May 2011 to discuss the crises in southern Sudan and Libya with then leader Goodluck Jonathan.

Buhari has vowed to destroy Boko Haram , and replaced his military leaders earlier in August, ordering his new chiefs-of-staff to end the jihadist bloodshed within three months.

The military under Jonathan was heavily criticised for poor handling of the insurgency and its failure to free more than 200 schoolgirls abducted from the northeastern town of Chibok in April last year.

The "Bring Back Our Girls" campaign marks 500 days on Thursday since the girls were captured.