Sinn Féin leader Mary Lou McDonald has criticised the "collective punishment of Palestinian people" in Gaza.
It came as the United Nations Palestinian refugee agency, UNRWA, warned Gaza was becoming a "hell hole" and was on the "brink of collapse".
The UN has said it is "impossible" for Palestinians to move to the south of Gaza in the next 24 hours, after citizens were ordered to evacuate by Israel's military.
The evacuation order is the strongest hint yet from Israel that it is preparing to launch a ground offensive on Gaza almost a week after a Hamas attack.
Damage to the UNRWA compound in Gaza is seen on October 13th 2023. Image: UNRWAIt also began dropping leaflets written in Arabic into the Gaza Strip urging people in the north of the area to move south across the Wadi Gaza - a piece of coastal wetland with a river running through the middle.
Deputy McDonald said she believes there is worse to come.
"These are not defensive actions by Israel. These are crimes. On the world's watch," she added in a tweet.
Collective punishment of Palestinian people. A humanitarian crisis. The imminent threat and certainty of worse to come. These are not defensive actions by Israel. These are crimes. On the world's watch. #IsraelPalestineConflict #Palestine https://t.co/JxAwY3ickm
— Mary Lou McDonald (@MaryLouMcDonald) October 13, 2023
UNRWA has warned that over 423,000 people have already been displaced.
More than 270,000 have taken refuge in UNRWA shelters where basic food, medicine and support is provided "to retain dignity and a glimmer of hope," it said.
UNRWA has said it is "urgently seeking" US$104 million (€99.06m) to enable its multi-sectoral humanitarian response over the coming 90 days.
"The requested funds will cover the urgent immediate food, non-food, health, shelter and protection needs of up to 250,000 persons seeking safety in UNRWA shelters across the ravaged Gaza Strip and another 250,000 Palestine refugees within the community," it explained.
A boy with a cat in the damaged Al Shati Refugee Camp in the Gaza Strip on October 13th, 2023. Image: UNRWA/Mahmoud AjjourUNRWA Commissioner-General Philippe Lazzarini said civilians in Gaza have nowhere to go.
"What is unfolding is already an unprecedented humanitarian tragedy," he said.
"Whatever the circumstances are, rules apply in times of conflict and this one is no exception.
"Aid to civilians who have nowhere to flee must be immediate: water, food, medicine.
"It is of utmost urgency that access to humanitarian assistance and protection be upheld for all civilians," he added.
Israel has said it is targeting the militant group Hamas after it carried out a wave of attacks in the country, including at a music festival which saw an Irish-Israeli woman killed.
Buildings destroyed in Gaza on October 7th, 2023. Image: UNRWAIsrael said a total of 1,300 of its people have died since Saturday's raids, while at least 500 children and 276 women are among the estimated 1,537 Palestinians who have died with more than 6,000 wounded, the Gaza health ministry has said.
Human Rights Watch has said Israel used the chemical white phosphorus in Gaza and Lebanon, claiming it had verified footage from October 10th and 11th, respectively, showing multiple airbursts of artillery-fired white phosphorus over the Gaza City port and two rural locations along the Israel-Lebanon border.
It said the weapon's use "violates the international humanitarian law prohibition on putting civilians at unnecessary risk".
White phosphorus ignites when exposed to oxygen and continues to burn until it is deprived of oxygen or exhausted.
It can create intense heat of about 815C and can burn people, thermally and chemically, down to the bone as it is highly soluble in fat.
