Hamas has said it will execute an Israeli hostage in response to any new Gaza Strip bombings that come without a pre-warning – moments after a senior figure from the group told Sky News civilian captives would be treated “humanely”.

Doctor Basem Naim, Hamas’s head of political and international relations, also claimed to Sky’s Mark Austin that the group has not “killed any civilians” since its militants crossed the border from the Gaza Strip and launched a surprise attack on Israel on Saturday.

Asked how many hostages the militant group had taken and brought into the Strip, Dr Naim said he couldn’t confirm an exact number, but said they would be treated in a “humane way, a proper way”.

He said he can “100% guarantee” they will be safe and orders have been given to fighters to not kill or harm elderly people, women or children.

However, moments later another Hamas official said it would begin executing Israeli civilian captives in return for any Israeli bombing of civilian houses in the Gaza Strip without warning.

Sky News understands Qatar is mediating between sides for an exchange of civilian hostages, including women and children.

The attack on Saturday morning took Israel by surprise.

Prime Minister Benjamin Netanyahu said the attack would “change the Middle East”.

“I know you have been through terrible and difficult things,” he told authorities from the south of the country. “What Hamas will go through will be difficult and terrible.

“We have only just begun.”

More than 1,000 Palestinian targets hit – follow live updates

His comments came before at least 100 bodies were recovered from Be’eri, a community in southern Israel that was seized by Hamas, rescue workers have told TV channels in the country.

Earlier on Monday, rescuers said they had recovered at least 260 bodies from the site of a music festival in Israel that was attacked by Hamas during its incursion.

However, Dr Naim had earlier told Sky News: “We have not killed any civilians.”

He added that he did not consider anyone “carrying guns” as a civilian.

Dr Naim said: “We have responded to 75 years of occupation, we have responded to 17 years of a suffocating siege, the silent killing of 2.3 million Palestinians.”

He says people in the Gaza Strip, controlled by Hamas, are “always under attack” and “never safe”.

“We are living in the biggest open-air prison. You have to choose either to die by F-35 or F-16 or to die silently of malnutrition,” he added.

The two days of violence between Israel and Hamas has reportedly left at least 900 Israelis and 500 Palestinians dead so far.

Meanwhile, Israel has also ordered a “complete siege” of the Gaza Strip, the Palestinian territory governed by Hamas, leaving it with no food, electricity or fuel.

United Nations Secretary-General Antonio Guterres later said he was “deeply distressed” by Israel’s decision to impose the siege.

It comes as Israeli forces continue to fight Hamas militants in two locations inside Israel after formally declaring war on the Palestinian militant group.

Around 300,000 reservists have been called up by the Israeli military as defence minister Yoav Gallant said his country was “battling beastly people” after Hamas launched its surprise attack from Gaza on Saturday.

Israeli authorities said a “huge rocket barrage” was fired at the country on Monday, with media in the country reporting that it came from Hamas-controlled areas.

Meanwhile, the Israeli military said it had hit more than 1,000 targets inside the Gaza Strip on Monday.

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Doron Spielman, a spokesperson for the Israel Defence Forces (IDF), said there is still active fighting with Hamas militants “constantly trying to cross over” the border.

“We expect this to continue because there’s a massive number of jihadis that have been trained to do this,” he told NBC.

This is despite Israeli troops having regained control in some areas of southern Israel.

The IDF has also said it is conducting strikes inside Lebanese territory. It said details would follow but provided no more information.

The strikes come after a number of reports of firefights on the border with Lebanon, with the IDF claiming it had killed “a number” of “infiltrators”.

The Lebanon-based Hezbollah group, which is backed by Iran, said it had not attempted any incursion.

More than 10 Britons are feared dead or missing in Israel, Sky News understands.

At least nine Americans have been confirmed dead with a further three missing.