Foreign Minister Israel Katz insisted that "Hamas rule will be crushed" and accused Palestinian president Mahmud Abbas, whose Fatah faction signed the deal, of embracing the group whose October 7 attacks triggered the war.
Any involvement by the Islamist militant group in the post-war governance of Gaza is anathema to the United States as well as Israel.
Israeli Prime Minister Benjamin Netanyahu is in Washington to address a joint session of Congress and has vowed to continue the Gaza war until Hamas is destroyed.
The diplomatic spat came as Israel hammered Gaza, including the southern city of Khan Yunis, where it had ordered a partial evacuation of civilians.
Chinese Foreign Minister Wang Yi hosted senior Hamas official Musa Abu Marzuk, Fatah envoy Mahmud al-Aloul and emissaries from 12 other Palestinian factions.
Hamas and Fatah are long-term rivals and fought a brief but bloody war in 2007 in which the Islamists seized control of Gaza.
Fatah continues to dominate the Palestinian Authority which has limited administrative control over urban areas of the Israeli-occupied West Bank.
The text of the deal outlined plans for "a temporary national unity government by agreement of the Palestinian factions" which would "exercise its authority and powers over all Palestinian territories" -- the Gaza Strip as well the West Bank, including Israeli-annexed east Jerusalem.
China, which last year brokered a deal restoring relations between regional rivals Iran and Saudi Arabia, hailed the agreement as a commitment to "reconciliation".
But Katz said Abbas "embraces the murderers and rapists of Hamas".
He also rejected any role for the Palestinian Authority in Gaza, saying "Abbas will be watching Gaza from afar".
'Gaza is dead'
On the ground, Israel pressed on with the war in Gaza. Hours after it ordered civilians to evacuate parts of Khan Yunis, including areas that had been declared part of a humanitarian safe zone, its jets pounded the city.
The Gaza health ministry said that 73 people had been killed and more than 200 wounded in the area, while thousands fled.
The Israeli military did not comment on the toll when asked by AFP. But in a statement, the military said its fighter jets and tanks "struck and eliminated terrorists in the area".
On Tuesday, it said its jets struck "over 50 terror infrastructure sites" as part of the Khan Yunis operation.
Hassan Qudayh, a resident forced to evacuate, said: "Gaza is over, Gaza is dead, Gaza has gone. There is nothing left, nothing".
More than nine months of war have obliterated much of the territory's healthcare capacity, with what remains under immense pressure.
Mohammed Zaqout, director of Nasser Hospital in Khan Yunis told AFP: "There is no space for more patients. There's no space in the operating theatres. There is a lack of medical supplies, so we cannot save our patients."
The World Health Organization said up to 14,000 people needed medical evacuation from Gaza and that it was "extremely worried" that diseases could cause more deaths than war injuries after poliovirus was detected in the territory's sewage.
The war was sparked by Hamas's October 7 attack on southern Israel which resulted in the deaths of 1,197 people, mostly civilians, according to an AFP tally based on Israeli figures.
The militants also seized 251 hostages, 116 of whom are still in Gaza, including 44 the Israeli military says are dead.
Israel's retaliatory military campaign in Gaza has killed at least 39,090 people, also mostly civilians, according to figures from the Hamas-run territory's health ministry.
- Landmark speech -
Beijing's dealmaking came with Netanyahu in Washington.
Netanyahu will deliver a landmark speech to Congress on Wednesday amid unprecedented strains between Israel and its ally.
The Israeli premier has resisted pressure from the administration of President Joe Biden to accept a truce, which far-right members of his coalition strongly oppose.
Biden, who will meet Netanyahu on Thursday, vowed to continue working to find a solution during his final six months in office, after announcing his withdrawal from the US presidential race.
Talks aimed at securing a truce are set to continue with an Israeli delegation due to travel to Doha on Thursday, a source with knowledge of the talks said.
Egypt, Qatar and the United States have been working to secure a ceasefire and hostage release deal between Israel and Hamas.