Hamas said Wednesday it would not bow down to threats from Israel and the United States, which demanded the group release hostages this weekend or face a return to war in Gaza.
Mediators Qatar and Egypt were pushing to salvage the ceasefire agreement, a Palestinian source and a diplomat familiar with the talks told AFP, while Hamas said its top negotiator was in Cairo.
The truce has largely halted more than 15 months of fighting and seen Israeli captives released in batches in exchange for Palestinians in Israeli custody.
But the deal, currently in its 42-day first phase, has come under increasing strain.
The warring sides, which have yet to agree the next phases of the truce, have traded accusations of violations, spurring concern that the violence could resume.
Hamas spokesman Hazem Qassem said on Wednesday that Israel was "evading the implementation of several provisions of the ceasefire agreement", warning that hostages would not be released without Israeli compliance with the deal.
"Our position is clear, and we will not accept the language of American and Israeli threats," said Qassem, after Israeli Prime Minister Benjamin Netanyahu threatened to "resume intense fighting" if hostages were not released by Saturday.
His threat echoed US President Donald Trump, who said on Monday that "hell" would break loose if Hamas failed to release "all" Israeli hostages by then. Netanyahu did not specify whether he was referring to all captives.
Hamas has said it would postpone the next hostage, release scheduled for Saturday, citing Israeli violations, and later insisted it was "committed to the ceasefire".
Hamas accused Israel of failing to meet its commitments under the agreement, including on aid, and cited the deaths of three Gazans at the weekend.
Hamas said in a statement that a delegation headed by its top negotiator and Gaza chief Khalil al-Hayya "arrived in Cairo and began meetings with Egyptian officials", and was monitoring "the implementation of the ceasefire agreement".
A senior Hamas official told AFP the delegation "will discuss ways to end the current crisis".
A diplomat familiar with the talks told AFP that mediators were engaged with both Israel and Hamas to resolve the dispute and ensure the implementation of the agreement.
A Palestinian source said earlier that Egypt and Qatar were "working intensively" to "resolve the crisis, compel Israel to implement the humanitarian protocol... and begin negotiations for the second phase".
UN chief Antonio Guterres has urged Hamas to proceed with the planned release and "avoid at all costs resumption of hostilities in Gaza".
The armed wing of Palestinian group Islamic Jihad, which has fought alongside Hamas in Gaza, said that the fate of the Israeli hostages it was holding was "directly tied to Netanyahu's actions".
In southern Gaza's Khan Yunis, 48-year-old Saleh Awad told AFP he felt "anxiety and fear", saying that "Israel is seeking any pretext to reignite the war... and displace" the territory's inhabitants.
Hours after Hamas's armed wing, the Ezzedine al-Qassam Brigades, said it was postponing Saturday's hostage release, Trump said that "if all of the hostages aren't returned by Saturday 12 o'clock... all bets are off and let hell break out."
Trump had earlier proposed taking over the Gaza Strip and moving its more than two million residents to Jordan or Egypt -- a plan experts say would violate international law but which Netanyahu called "revolutionary".
The US president reaffirmed his hostage release deadline on Tuesday when hosting Jordan's King Abdullah II, who on social media "reiterated Jordan's steadfast position against the displacement of Palestinians".
Egypt said it planned to "present a comprehensive vision for the reconstruction" of the Palestinian territory which ensures residents remain on their land.
The Israeli military said it had reinforced its troops around Gaza, and on Wednesday conducted an air strike targeting suspected weapons smugglers in the territory.

A Palestinian amputee walks near destroyed buildings in Beit Hanun in the northern Gaza Strip on Wednesday. AFP