The Israeli military has ordered Palestinians to leave a number of neighbourhoods in the southern city of Khan Younis, including areas that had been designated by the military as part of a humanitarian zone.

Palestinian civil defence in the territory estimated that 400,000 people sheltering in the city were affected by the order, which included the eastern part of al-Mawasi, a sandy strip of land without infrastructure where Palestinians have sought shelter in tent encampments in recent months.

The military said it planned to start an operation against Hamas militants in Khan Younis and part of Al Mawasi, claiming they used the area to launch rockets at Israel.

“We were displaced from the eastern regions, they called us to evacuate, we took our children and left,” Osama Qudeih told the Associated Press (AP). “There was no safe place left in the Gaza Strip … We went out walking in the streets, not knowing where to go.”

Another woman collapsed in exhaustion after saying it was her seventh or eighth displacement. “Every day we are displaced,” Kholoud al-Dadas told AP as she clutched her children. “Where are the countries? Where is the world, where are the presidents, where are they? Come and see how we are, our children, and what is happening to us.”

The military said it was adjusting the boundaries of the designated humanitarian zone in al-Mawasi to keep the civilian population away from areas of combat.

Gaza health officials said at least 37 people had been killed and 120 wounded in attacks on and around Khan Younis, and that more casualties were likely to be buried under rubble or left on roadsides because ambulances had been unable to reach them.

The local Wafa news agency reported that a series of fierce bombardments began immediately after Israeli forces dropped leaflets telling people to evacuate. Columns of smoke were visible over the destroyed rooftops of the city.

Wounded people poured into Nasser hospital in Khan Younis as the facility appealed for blood donations.

Much of Khan Younis was destroyed by fierce fighting in the months before Monday’s strikes, but hundreds of thousands of people returned to seek shelter in tents in the eastern part of the city after they were displaced from other parts of Gaza. Images from Khan Younis showed Palestinians fleeing the area in cars and on donkey carts, using whatever means they could find to escape.

Palestinians in the territory, the UN and other international relief agencies have stressed that no safe place remains in Gaza, and that areas designated as humanitarian safe zones are still targeted. A strike on Al Mawasi earlier this month killed at least 90 people and injured hundreds when Israeli forces said they had targeted the head of Hamas’s military wing.

More than 39,000 people have been killed in Israel’s onslaught in Gaza since 7 October last year, when Hamas militants attacked Israeli territory, killing 1,200 people and taking about 250 hostage. The Palestinian health ministry also said more than 89,000 had now been injured after a series of intense Israeli attacks across the territory in recent weeks.

The head of the UN agency for Palestinian refugees (Unrwa), Philippe Lazzarini, said Israeli forces had fired repeatedly on a UN convoy headed to Gaza City on Monday.

“Heavy shooting from the Israeli forces at a UN convoy heading to Gaza City,” he said, adding that one vehicle had been struck with five bullets while waiting ahead of an Israeli checkpoint near the Gaza River, causing severe damage.

“While there are no casualties, our teams had to duck and take cover … The teams were travelling in clearly marked UN armoured cars and wearing UN vests,” he said, adding that the convoy had coordinated and approved its journey with the Israeli authorities. Spokespeople for the Israel Defense Forces did not respond to a request for comment on the incident.

The Jordanian foreign minister, Ayman Safadi, commented on Lazzarini’s description of the attack, calling it a war crime.

“Gaza has not only become a graveyard for children. It has become a graveyard for international law, a shameful stain on the whole international order,” he said.

Associated Press and Reuters contributed to this report