His comments came after Palestinian militant group Hamas called for international pressure on Israel to enter the next phase of the ceasefire, as negotiations were resuming in Cairo.
The truce's first phase is due to expire Saturday morning, after largely halting more than 15 months of bombardment and fighting in the Gaza Strip between Israel and Hamas.
"The ceasefire and hostage release deal must hold. The coming days are critical. The parties must spare no effort to avoid a breakdown of this deal," Guterres said in New York.
The truce led to the exchange of hostages held by Hamas in return for Palestinians held in Israeli jails. It also enabled greater aid flows into the territory, where more than 69 percent of buildings were damaged or destroyed, almost the entire population was displaced, and widespread hunger occurred because of the war, according to the United Nations.
But talks on a second phase had been delayed.
They were supposed to begin 16 days into the first stage, and an analyst, Max Rodenbeck, of the International Crisis Group think tank, said the second phase -- aiming to bring a permanent end to the war -- cannot be expected to start on Saturday.
"But I think the ceasefire probably won't collapse also," he said.
The preferred Israeli scenario is to free more hostages under an extension of the first phase, rather than a second phase, Defence Minister Israel Katz said Thursday.
Hamas said in a statement that "with the end of the first phase of the ceasefire", the group "affirms its full commitment to implementing all the provisions of the agreement in all its stages and details".
The group called for global pressure on Israel "to immediately enter the second phase of the agreement without any delay".
The State Information Service of mediator Egypt said in a statement on Thursday: "The relevant parties have begun intensive talks to discuss the next phases of the truce agreement, amid ongoing efforts to ensure the implementation of the previously agreed understandings".
Israeli Prime Minister Benjamin Netanyahu on Thursday "instructed the negotiation delegation to depart for Cairo", his office said shortly after Hamas handed over the remains of Tsachi Idan and three other hostages under the truce, in exchange for hundreds of Palestinians in Israeli custody.
- Ramadan begins -
In Gaza and throughout the Muslim world, this weekend is also the start of the Muslim holy month of Ramadan.
The Gaza war began with Hamas's attack on Israel on October 7, 2023, which resulted in the deaths of 1,218 people, mostly civilians, according to an AFP tally of official figures.
The Israeli retaliation has killed more than 48,000 people in Gaza, according to the health ministry in the Hamas-run territory, figures the UN has deemed reliable.
In a relatively rare incident during the truce, Israel's military said an air strike on Friday targeted two "suspects" approaching troops in southern Gaza, as a hospital in Khan Yunis said it had received the body of one person killed "in a strike".
On Friday thousands of people lined the streets around the Israeli commercial hub Tel Aviv for the funeral of Idan, 49, whose body was among those returned to Israel a day earlier.
They held a memorial service at Bloomfield Stadium, home ground of Hapoel Tel Aviv, the football club Idan supported. Many stood in the stands, weeping and waving the club's red colours alongside the national flag.
Israeli authorities said Idan was "murdered while held hostage in Gaza".
- 'Shackled with chains' -
Israel's Prison Service said 643 inmates were released after Hamas returned the four hostages' bodies.
Among those freed was the longest-serving Palestinian prisoner in an Israeli jail, Nael Barghouti, who spent more than four decades behind bars including for the murder of an Israeli officer.
Hamas freed in stages 25 living Israeli hostages and returned the bodies of eight others under the truce's first phase.
Among those released was Eli Sharabi, now 53, who recounted his suffering in a televised interview.
"For a year and four months my legs were shackled with chains with very, very heavy locks that cut into your flesh," Sharabi said.
He spoke of intense hunger and food deprivation.
In return for the release of Sharabi and other captives, Israel was expected to free around 1,900 Palestinian prisoners in total during the first phase. Hamas officials told AFP that Israel had freed about 1,700 so far.
Gaza militants also released five Thai hostages outside the deal's terms.
Of the 251 hostages seized during Hamas's attack, 58 are still being held in Gaza, including 34 the Israeli military says are dead.
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