The Israeli military said on Wednesday it would hold fire unilaterally in limited areas of the Gaza Strip for four hours from 3 p.m. (1200 GMT) for humanitarian purposes, an army statement said.
Hamas described the truce as a media stunt because it only applied to some areas, Reuters reported.
"IDF [Israel Defence Forces] has authorized a temporary window in the Gaza Strip. The humanitarian window will not apply to the areas in which IDF soldiers are currently operating," the Israeli statement said.
Israeli shelling killed at least 15 Palestinians sheltering in a school in Gaza's biggest refugee camp earlier on Wednesday, the Gaza Health Ministry said. Some 3,300 Palestinians, including many women and children, were taking refuge in the building in Jabalya refugee camp when it came under fire around dawn, the United Nations Relief and Works Agency (UNRWA) said.
An Israeli military spokeswoman said militants had fired mortar bombs from the vicinity of the school and troops shot back in response. The incident was still being reviewed. It was not clear whether the humanitarian ceasefire included Jabalya.
The Health Ministry has said 1,287 Palestinians, mostly civilians, have been killed since Israel began its offensive in the Hamas-controlled enclave on July 8 with the declared aim of halting cross-border rocket fire.
On the Israeli side, 53 soldiers and three civilians have been killed. In ground operations launched 10 days later, Israel said its main mission is to locate and destroy tunnels that Gaza militants have built under the frontier and have used to launch attacks inside Israel.
Israeli Prime Minister Benjamin Netanyahu was due to review progress with his security cabinet later on Wednesday, and a Palestinian delegation was expected in Cairo for discussions on a permanent ceasefire.
Hamas spokesman Sami Abu Zuhri said of the truce: "The declared calm ... is worthless because it excludes the hot areas along the Gaza borders and we will not be able to benefit from it to evacuate the wounded in those area."