TEL AVIV – The parents of a slain Israeli soldier whose body is being held captive by Hamas are demanding the United Nations and U.S. “take responsibility” to recover his remains after the Gaza-based terror group abducted their son during a U.S. and UN-brokered ceasefire in the 2014 summer conflict.

Leah and Simcha Goldin, whose son Hadar was killed in the Gaza war, headed to New York with Hadar’s twin brother Tzur to attend the Wednesday opening of an exhibit of their son’s artwork at UN headquarters.

The Goldins also used the opportunity to lobby for the return of their son’s body for proper burial, marking their fourth trip to the U.S. for this purpose.

“The basic message is that Hadar was killed two hours after a ceasefire was declared. Hamas ambushed the Israelis while they were not shooting because they were under the ceasefire protocol,” Leah Goldin told Arutz 7 news.

“We are expecting that the UN and United States, who brokered the ceasefire, take responsibility for the return of Hadar and Oron for burial in their homeland,” she added, referring to Oron Shaul, another soldier killed during the war whose remains are being held by Hamas.

The Goldins’ trip coincides with the 71st meeting of the UN General Assembly. They are set to meet with Israeli officials and Jewish-American leaders among others.

The Goldins believe that pressure from the U.S. and UN exerted on Hamas will result in the return of Hadar’s body.

“We know the UN can pressure Hamas until the latter can no longer bear the cost of continuing to hold our son’s body,” Leah Goldin said.

“The whole world is struggling now to rebuild Gaza, and we keep saying that the return of Hadar and Oron is the basic humanitarian value according to all religions, including Islam,” she said.

“We’re looking to find the linkage between our basic humanitarian value to the world issue of humanitarian aid, and by creating this linkage, make it clear that there are no ‘free lunches.’ If you want the world to assist you, then you have to adhere to our basic values and not violate them.”

In a separate interview, Leah Goldin said that Israel was a “key actor” in returning his remains.

“Israel has to define the price to Hamas for keeping bodies,” she said, noting that the government last month allowed for the transfer of salaries to Hamas. “We do not have a problem with that and want them to have normal lives, but Hadar’s body should be returned first.”

The art exhibit’s opening was attended by Israel’s ambassador to the UN, Danny Danon, who has been assisting the Goldins in the campaign for their son’s body.

“Hadar was killed and kidnapped by despicable terrorists during a UN sponsored ceasefire,” Danon said. “He was not only a brave solider, but a talented artist taken before his time. I will continue to work tirelessly for his return.”