- Israel says 50 trucks have been deployed into northern Gaza
Qatar is in ongoing talks with Israel and Hamas to resolve the delay in hostage releases "as soon as possible," an official briefed on the situation told Reuters.
Egypt was also trying to resolve the delay, a Palestinian official familiar with the matter who did not want to be named told Reuters.
Israel said it was honouring the truce. Earlier it said 50 aid trucks carrying food, water, shelter equipment and medical supplies had deployed to northern Gaza under UN supervision, the first significant aid delivery to northern Gaza since the start of the war.
The statement by the al-Qassam Brigades punctured hopes of a smooth second day of hostage and prisoner releases after 13 Israeli women and children were freed by Hamas on Friday.
Some 39 Palestinian women and teenagers were released from Israeli jails under the truce accord mediated by Qatar.
Hamas' al-Qassam Brigades said Israel was failing to respect the terms of the Palestinian prisoner release agreed as part of the hostage deal. Qadura Fares, the Palestinian commissioner for prisoners, said Israel had not released detainees according to seniority, as was expected.
Israeli army spokesperson Olivier Rafowicz told French television Israel was strictly honouring the terms of the truce, and said the military had carried out no attacks or offensive operations in Gaza on Saturday.
"There is a delay...in the release of the hostages. I do not want in any way to get into what the armed wing of Hamas is saying," Rafowicz told BFM TV. "This situation is obviously being managed at the highest level in Israel."
The Israeli army spokesman said earlier Israel had been expecting another 13 hostages to be set free on Saturday, with 39 Palestinian prisoners also to be released - barring last-minute changes.
A total of 50 hostages are to be exchanged for 150 Palestinian prisoners over four days under the truce.
Hours before the al-Qassam Brigades statement, Egypt, which controls the Rafah border crossing through which aid supplies have resumed into southern Gaza, said it had received "positive signals" from all parties over a possible truce extension.
Diaa Rashwan, the head of Egypt's State Information Service (SIS), said in a statement that Cairo was holding extensive talks with all parties to reach an agreement which would mean "the release of more detainees in Gaza and Palestinian prisoners in Israeli jails."
Israel has said the ceasefire could be extended if Hamas continues to release hostages at a rate of at least 10 per day. A Palestinian source has said up to 100 hostages could go free.
