UN Secretary-General Ban Ki-moon demanded Friday that an Israeli soldier captured in Gaza be released immediately and condemned "in the strongest terms" the reported ceasefire violation by Hamas.
Ban called on Israel and Hamas to show restraint and return to the 72-hour truce that collapsed after two soldiers were killed and a third reportedly captured near the southern city of Rafah.
"The secretary-general demands the immediate and unconditional release of the captured soldier," his spokesman said in a statement.
Ban was "shocked and profoundly disappointed" by the renewed violence and warned that if reports of the attack on Israeli soldiers were confirmed "this would constitute a grave violation of the ceasefire," the statement added.
"Such moves call into question the credibility of Hamas's assurances to the United Nations."
Israel and Hamas traded accusations over the collapse of the ceasefire amid renewed fierce fighting that left scores dead.
The United Nations has repeatedly called for a humanitarian truce to give respite to civilians from the fighting that has left nearly 1,500 dead on the Palestinian side, mostly civilians, and 63 Israeli soldiers and three civilians on the other.
- Uncertainty over Egypt-brokered talks -
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The UN's top political affairs chief poured cold water on prospects for peace talks brokered by Egypt after the collapse of the 72-hour truce that was to lay the groundwork for negotiations.
"The chance for success of these talks rested on quiet, rested on the 72-hour ceasefire," said Jeffrey Feltman, Under Secretary General for Political Affairs.
"Right now we have escalation. I myself have a hard time envisaging how these talks would work right now."
Egypt said the invitation to Israeli and Palestinian delegations was still open and Palestinian president Mahmud Abbas said his team would travel to Cairo for the talks on Saturday.
UN officials pressed for a return to the ceasefire, but acknowledged that the conflict had escalated and that the abduction of the Israeli soldier had virtually killed prospects for a truce.
"I hope that we can get back to that (a truce) but it is going to be extremely difficult given the situation we see in the Gaza Strip now especially with the capture of the Israeli soldier," said Feltman.
Feltman stressed that the United Nations had received firm assurances from both sides "after long, hard, tough" negotiations that they would suspend fighting for 72 hours.
Those talks involved US Secretary of State John Kerry and key players from Qatar and Turkey, who are in contact with Hamas, as well as Egypt and Saudi Arabia.