The veteran Palestinian-American journalist, 51, was fatally shot while reporting on an Israeli military operation in the West Bank town of Jenin.
Her producer, fellow journalist Ali Samoudi, also was wounded, according to media reports.
The Secretary-General António Guterres issued a statement saying he was “appalled by the killing”.
“He sends his heartfelt condolences to the family of Ms. Abu Akleh and wishes a quick recovery to fellow journalist Ali Samoudi”, the statement continued.
“He calls on the relevant authorities to carry out an independent and transparent investigation into this incident and ensure that those responsible are held accountable.
“The Secretary-General condemns all attacks and killings of journalists and stresses that journalists must never be the target of violence. Media workers should be able to carry out their work freely and without harassment, intimidation or the fear of being targeted.”
Audrey Azoulay, head of the UN Educational, Scientific and Cultural Organization (UNESCO), issued a statement condemning her killing.
Violation of international law
Ms. Abu Akleh was shot “despite the fact that she was wearing a jacket with the word ‘press’ written on it”, she said.
“The killing of a clearly identified press worker in a conflict area is a violation of international law. I call on the relevant authorities to investigate this crime and bring those responsible to justice.”
Ms. Azouley recalled that UNESCO works to raise awareness about the need to protect journalists, notably through the UN Plan of Action on the Safety of journalists and the Issue of Impunity.
UNESCO is also the lead agency for World Press Freedom Day, observed annually on 3 May.
Not a target
The UN Special Coordinator for the Middle East Peace Process took to Twitter to express his strong condemnation.
Tor Wennesland offered his deepest condolences to the family of the slain journalist and wished a speedy recovery to her injured colleague.
“I call for an immediate and thorough investigation and for those responsible to be held accountable. Media workers should never be targeted,” said Mr. Wennesland.
‘Impunity must end’
His Deputy, Lynn Hastings, who is also the UN Humanitarian Coordinator for the Occupied Palestinian Territory, highlighted the risks journalists face in reporting the news.
Ms. Abu Akleh was killed as the UN was marking World Press Freedom Day in Gaza, thus “showing dangers journos face every day”, she wrote on Twitter. “Prompt investigations must be conducted for accountability.”
The UN human rights office, OHCHR, said it was appalled by the killing.
“Our Office is on the ground verifying the facts,” OHCHR tweeted. “We urge an independent, transparent investigation into her killing. Impunity must end.”
UN chief appalled
Secretary-General António Guterres was appalled by the killing and echoed the call for the relevant authorities to conduct an independent and transparent investigation, according to a statement issued by his Spokesperson.
Mr. Guterres also condemned all attacks and killings of journalists, stressing that they must never be the target of violence.
“Media workers should be able to carry out their work freely and without harassment, intimidation or the fear of being targeted,” the statement said.
The UN chief reiterated his firm conviction that a free press is essential for peace, justice, sustainable development and human rights.