UN Secretary-General Antonio Guterres has said that humanitarian aid must be permitted into Gaza, as Israeli leaders ordered a "complete siege" of the strip.

Guterres told reporters on Wednesday that about 220,000 Palestinians are now sheltering in 92 facilities belonging to the UN Relief and Works Agency for Palestine Refugees in the Near East, or UNRWA.

He said that to ensure the protection of civilians at all times, UN premises, hospitals, schools and clinics must never be targeted.

Guterres added, "Crucial life-saving supplies -- including fuel, food and water -- must be allowed into Gaza. We need rapid and unimpeded humanitarian access now."

He stressed, "There is no time to lose. Every moment counts," calling on all parties concerned to take action to prevent a worsening of the situation.

Meanwhile, US Defense Secretary Lloyd Austin told reporters in Belgium on Wednesday that Washington will continue sending military aid to Israel.

Austin said that the US government will be providing more Iron Dome interceptors so that Israel can protect its citizens and cities from rocket attacks by the Palestinian Islamist group Hamas.

He added, "We will continue to ensure that Israel has what it needs to keep itself and its citizens safe."