"The Secretary-General (Antonio Guterres) continues to call for an immediate and independent investigation into these events and for the perpetrators to be held to account," UN spokesman Stephane Dujarric told reporters.
The Israeli military said its forces had opened fire on a group of people on Tuesday they viewed as a threat after they left a designated access route near a distribution centre in Rafah and approached their positions.
It added it was still investigating what had happened.
The deaths came hours after Israel said three of its soldiers had been killed in fighting in the north of the Gaza Strip as its forces pushed ahead with a months-long offensive against Hamas militants that has laid much of the enclave to waste.
Reuters could not independently verify the reports in the northern and southern parts of the Gaza Strip.
An International Committee of the Red Cross spokesperson said its field hospital in Rafah had received 184 casualties, adding that 19 of those were dead upon arrival, and eight died of their wounds shortly after.
Video showed injured people, including at least one woman, being rushed to a medical centre on carts drawn by donkeys.
Health officials said at least 18 more Palestinians were killed in other military strikes in the territory on Tuesday.
The United Nations human rights office in Geneva said on Tuesday the impediment of access to food relief for civilians in the Gaza Strip might constitute a war crime and described attacks on people trying to access food aid as "unconscionable".
The head of the UN agency, Volker Turk, urged a prompt and impartial investigation into the killings.
Israeli government spokesman David Mencer denied that civilians had been targeted.
"The IDF is doing everything in its power to allow Gazans to get to the humanitarian aid. The IDF is not preventing the arrival of Gazans at humanitarian aid sites. Indeed, we are encouraging it," Mencer said.
The ten elected members of the UN Security Council asked for the 15-member body to vote on Wednesday on a draft resolution that demands "an immediate, unconditional and permanent ceasefire in Gaza respected by all parties," diplomats said.
The draft text, seen by Reuters, also demands the release of all hostages held by Hamas and others, and the immediate lifting of all restrictions on the entry of humanitarian aid into the Gaza Strip and its safe and unhindered distribution at scale, including by the UN throughout the enclave.
A resolution needs nine votes in favour and no vetoes by the permanent members - the United States, Russia, China, the United Kingdom or France - to pass.
