At least 19 people have been killed in an overnight Israeli strike in the designated humanitarian zone in southern Gaza, the Hamas-run health ministry says.

Witnesses said the strike obliterated an area crowded with tents for displaced Palestinians in al-Mawasi, south-west of Khan Younis, leaving huge craters in the sand.

“The bombing was incredibly intense. People were thrown into the air,” one displaced man told the BBC. “You can’t imagine the devastation.”

The Israeli military said its aircraft attacked what it called “a number of senior Hamas terrorists” operating there – a claim Hamas denied.

The military also disputed the initial death toll put out by the Hamas-run Civil Defence authority, which reported that rescue teams had recovered more than 40 bodies.

Hundreds of thousands of people from other areas of Gaza are living in dire conditions in al-Mawasi after being told by Israel to evacuate there for their own safety.

The UN says the humanitarian zone spans only 41 sq km (16 sq miles) and lacks critical infrastructure and basic services, while aid provision is limited due to access and security issues.

Eyewitnesses said large explosions rocked al-Mawasi shortly after midnight local time on Tuesday (21:00 GMT on Monday).

Khaled Mahmoud, a volunteer for a charity who lives near the site of the strike, told the BBC that he and other volunteers rushed to help but were stunned by the scale of the destruction.

“The strikes created three craters 7m [23ft] deep and buried more than 20 tents,” he said.

Aya Madi, a displaced mother of seven from the southern city of Rafah, later told a freelance journalist working for the BBC: “We woke up to nothing but sand and fire.

“My children were screaming, and the tent collapsed on them. I pulled them out from under the rubble.

“I held my two-month-old son, thinking he was dead, covered in sand, barely breathing. I washed him and thanked God he was still alive.”

She said all of those killed were civilians, adding that there was “not a single resistance fighter”.

“All that remains is dust and ashes,” she added. “Some of [the casualties] were torn in parts, other they had to dig to find, some were found in people’s houses… The scene is terrifying.”

Another woman, Aisha Nafi al-Shaeri, said: “It was all tents of displaced people. And now everything is destroyed.”

“They told people to stay here, but there is no safe area. Those people had nothing, and they were sleeping.”

“They didn’t warn anyone. Everyone was sleeping, and suddenly they started shelling.”

The Civil Defence’s operations director said overnight that more than 40 people were killed and more than 60 others were injured.

Later on Monday, an official at the nearby Nasser hospital in Khan Younis told the BBC that 13 people killed in the strike had been brought there.

In the afternoon, the Hamas-run health ministry said in a statement that hospitals had received a total of 19 bodies and more than 60 wounded people, some of whom were in a serious condition. It did not say how many were men, women or children.

Rescuers were unable to reach a number of other victims still buried under sand and rubble, or on roads, the ministry added.

In a statement, the Israel Defense Forces (IDF) said aircraft had conducted “a precise strike on a number of senior Hamas terrorists who were operating within a command and control centre embedded inside the humanitarian area”.

They included Samer Abu Daqqa, head of Hamas’s aerial unit, and Osama Tabesh, head of the observation and targets department in Hamas’s military intelligence headquarters, it added.

“Prior to the strike, extensive intelligence gathering was conducted, as well as continuous aerial surveillance in the hours leading up to the strike, which confirmed the presence of the terrorists in the area alongside additional terrorist operatives,” it said, adding that casualty numbers put out by Hamas-run authorities did “not align” with its information.

The IDF accused Hamas of embedding its operatives and military infrastructure in the humanitarian zone and using civilians as human shields.

A Hamas statement denounced the strike on al-Mawasi as a “heinous massacre” and rejected the IDF’s claim that it had a command centre there.

“This is a clear lie that aims to justify these ugly crimes. The resistance has denied several times that any of its members exist within civilian gatherings or use these places for military purposes,” it said.

UN Middle East peace envoy Tor Wennesland condemned the strike on a densely-populated area where displaced people were sheltering.

“I underline that international humanitarian law, including the principles of distinction, proportionality, and precautions in attack, must be upheld at all times,” he said. “I also emphasize that civilians must never be used as human shields.”

He also urged Israel and Hamas to immediately agree a deal that would for a ceasefire and the release of the remaining hostages in Gaza.

The Israeli military launched a campaign to destroy Hamas in response to the group’s unprecedented attack on southern Israel on 7 October, in which about 1,200 people were killed and 251 others were taken back to Gaza as hostages.

More than 40,980 people have been killed in Gaza since then, according to the territory’s Hamas-run health ministry.

An estimated 1.9 million people – more than 80% of the population – are estimated to have been displaced by the war, and some have had to flee as many as 10 times.