UN Secretary General Ban Ki-moon condemned "in the strongest terms" the reported ceasefire violation by Hamas and demanded that an Israeli soldier captured in Gaza be released immediately.
"The Secretary-General condemns in the strongest terms, the reported violation by Hamas of the mutually agreed humanitarian ceasefire which commenced this morning. He is shocked and profoundly disappointed by these developments," Ban's spokesman said in a statement.
The UN and US had announced yesterday that Israel and Hamas have agreed to an "unconditional humanitarian ceasefire" in Gaza for a period of 72 hours and that the Israeli and Palestinian delegations would immediately be heading to Cairo for negotiations with the Egyptian government on reaching a "durable ceasefire".
However, two Israel Defence Forces soldiers were killed and one taken captive, hours after the humanitarian ceasefire came into effect.
"This would constitute a grave violation of the ceasefire, and one that is likely to have very serious consequences for the people of Gaza, Israel and beyond. Such moves call into question the credibility of Hamas' assurances to the United Nations," Ban said demanding the "immediate and unconditional" release of the captured soldier.
Noting that the UN has no independent means to verify what happened on ground, Ban said he is deeply concerned about the resumption of Israeli attacks on Gaza and the killing of over 50 Palestinians this morning.
"Instead of giving both sides, especially Gazan civilians, a much needed reprieve to let them attend to their injured, bury their dead and repair vital infrastructure, this breach of the ceasefire is now leading to a renewed escalation," Ban's spokesman said.
"The Secretary-General urges both sides to show maximum restraint and return to the agreed 72-hour humanitarian ceasefire that tragically lasted such a brief period of time," he said.
Ban urged those with influence over the parties to do everything to convince them to observe the humanitarian ceasefire.
Artillery shelling by Israeli in the southern Rafah area killed dozens of people, taking the Palestinian death toll to over 1,500, mostly civilians.
Israel and Hamas have blamed each other for the collapse of the fragile ceasefire.