The reports came as mediators in Cairo said they were pressing on with efforts to salvage a fragile US-brokered ceasefire deal that has ended major clashes but left many key points unresolved.
Medics said two people were killed when an Israeli strike hit near a tent encampment in the Mawasi area of Khan Younis in the south of the enclave.
In the Jabalia refugee camp in northern Gaza, medics said three Palestinians, including an eight-year-old boy, were killed and others were wounded in an Israeli air strike that hit near a group of people digging a well. Later on Monday, an Israeli air strike near the Al-Aqsa Hospital in Deir Al-Balah in central Gaza Strip, killed one person and wounded three others, medics said, taking Monday's death toll to six.
The Israeli military did not immediately comment on the reports of the attacks or of its forces moving markers to expand the Yellow Zone under their control.
Israel and Hamas have repeatedly accused each other of violating the truce. Israeli strikes have killed more than 950 people since the truce, health officials say, while Israel says four soldiers were killed by militants during the same period.
Israeli troops still control more than 60 per cent of Gaza's territory, where they have ordered residents out and destroyed remaining buildings. Nearly the entire population of two million people now live in a tiny strip of land along the coast, mainly in makeshift tents or damaged buildings, under Hamas control.
The territory has been bombarded to ruins by Israel's two-year military assault that followed the 2023 Hamas attack on southern Israel. On Sunday, Prime Minister Benjamin Netanyahu reaffirmed he had directed Israel's military to expand its hold and take control of 70 per cent of the enclave.
"We are not allowing them to arm themselves or harm us, and we are also eliminating their senior commanders," he said in a speech.
Witnesses in the southern Gaza areas said the Israeli forces have in the past few days expanded the Yellow Zone, in some areas in eastern Khan Younis and northern Rafah where new markers and concrete blocks have been placed.
They added that new markers bring forces closer to areas populated with tents and displacement centres.
In Bani Suhaila town, in eastern Khan Younis, some people say they could see the tanks from their tents.
Three days into a new round of ceasefire talks in Cairo, leaders of Hamas and other Palestinian groups told mediators that Israel must end attacks to allow discussion on the second phase.
They demanded more flow of aid and goods into Gaza, and the retreat of Israeli forces to original October ceasefire lines.
A Hamas official told Reuters on Monday that Israel had so far refused to make commitments to mediators over any of the demands made by Hamas and the factions.
No agreement has been reached to implement a further US-backed plan for Israeli troops to withdraw, Hamas to disarm and Gaza to be rebuilt.
"Israel refuses to end attacks on civilians in Gaza, allow 600 trucks of aid and goods into Gaza as agreed and it continues to occupy more land every day," the official said.
