The attacks were carried out on two homes, where women and children were among the 18 dead, and a school housing displaced families, among other areas, according to Gaza medics.
Israel's military, which on Monday warned those in the southern Gazan city of Khan Younis to flee to the coast as it prepared for an "unprecedented attack", had no immediate comment.
Tuesday's strikes were carried out on Khan Younis and areas to the north, including Deir al-Balah, Nuseirat, Jabalia, and Gaza City, the medics said.
Israeli strikes have killed more than 500 people in the past eight days as the military campaign has intensified, they say.
Israel's military said on Monday it allowed five aid trucks into Gaza after a more than two-month blockade of food and other supplies.
The UN has long said Gaza, with a population of about 2.3 million, needs at least 500 trucks of aid and commercial goods every day.
Throughout the war, trucks with aid have waited weeks and months at Gaza's border to enter.
The war, now in its 20th month, has strained Israel's relations with much of the international community and those with its closest ally, the United States, now appear to be wavering.
The leaders of Britain, France and Canada warned on Monday they could take "concrete actions" against Israel if it did not stop military operations in Gaza and lift its restrictions on aid.
In a separate statement alongside the European Union and 20 other nations, the three countries warned that Gaza's population was facing starvation and that the UN and aid groups must be allowed to carry out their work independently.
Responding to the leaders' criticism, Israeli Prime Minister Benjamin Netanyahu said his country was engaged in a "war of civilisation over barbarism" and vowed it would "continue to defend itself by just means until total victory".
Under a heavily criticised US and Israeli-backed plan to deliver aid, a newly created Gaza Humanitarian Foundation aims to start work in Gaza by the end of May.
Israel's ground and air war has devastated Gaza, displacing nearly all its residents and killing more than 53,000 people, many of them civilians, according to Gaza health authorities.
The war erupted after Hamas-led militants attacked Israeli communities near Gaza's border on October 7, 2023, killing about 1200 people, mostly civilians, and seizing 251 hostages, according to Israeli tallies.
Israel's leadership has insisted that it can free the hostages and dismantle Hamas through force.
Netanyahu has said Israel aims to control the whole of Gaza.
Hamas has said it would release the hostages in exchange for an end to the war and the release of Palestinians in Israeli jails.
A new round of indirect ceasefire talks in Qatar between Israel and Hamas has produced no breakthrough.
with reuters
