Israel released 90 Palestinian prisoners on Monday after Hamas handed over three Israeli hostages, completing the first exchange of a truce aimed at pausing 15 months of conflict in Gaza.

The move was hailed by the United Nations, which is urging more much-needed aid to the devastated enclave.

The three female hostages released on Sunday were reunited with their families and will remain in hospital for several days — all in stable conditions, The Times of Israel reported.

Hours later, in the West Bank, the released Palestinian prisoners — 69 women and 21 teenage boys — left Ofer Prison on buses at around 1 am, with crowds celebrating their arrival in the nearby town of Beitunia.

As the cease-fire took effect on Sunday morning, thousands of displaced Palestinians, carrying tents, clothes and personal belongings, set off across the Gaza Strip to return home.

In Deir al-Balah central Gaza, Umm Hasan al-Buzom, 70, said she would even "crawl my way back home" if needed. "But we can't return for fear that the (Israeli) occupation forces might shoot at us."

The initial 42-day truce, brokered by Qatar, Egypt and the United States, calls for fighting to stop, aid to be sent into Gaza and 33 of the nearly 100 remaining hostages to go free over the six-week first phase in return for nearly 2,000 Palestinians in Israeli custody. It intends to pave the way for a permanent end to the conflict, but a second phase has yet to be finalized.

Fireworks were launched in celebration as buses carrying the 90 freed Palestinians arrived in Ramallah on the West Bank, where thousands of people were waving the Palestinian flags and filming the scenes on their phones.

Ahmed Abu Ayham, a 40-year-old from Gaza City, said that while the cease-fire may have spared lives, the losses and destruction made it no time for celebration. "We are in pain, deep pain and it is time to hug one another and cry," he said.

There is still no detailed plan in place to govern Gaza after the conflict, much less to rebuild it.

More than 47,000 Palestinians have since been killed in Israeli attacks. Nearly the entire 2.3 million population of Gaza is homeless. About 400 Israeli soldiers have also died.

China welcomes the Gaza ceasefire agreement coming into effect, Foreign Ministry spokeswoman Mao Ning said at a regular news briefing on Monday.

"We hope that the agreement will be fully and continuously implemented, and that a comprehensive and permanent cease-fire in Gaza will be achieved," she said.

"China will continue to work with the international community to promote peace and stability in the Middle East."

Removing obstacles

In a statement, UN Secretary-General Antonio Guterres welcomed the cease-fire and hostage release.

"We stand ready to support this implementation and scale up the delivery of sustained humanitarian relief to the countless Palestinians who continue to suffer. It is imperative that this cease-fire removes the significant security and political obstacles to delivering aid," he said.

Until the truce, Israel had complete control over the volume and nature of aid allowed into Gaza.

The UN has said that reconstruction could take more than 350 years if Gaza remains under Israeli blockade.

Using satellite data, it estimated last month that 69 percent of Gazan structures had been damaged or destroyed, including more than 245,000 homes. With more than 100 trucks working full-time, it would take 15 years just to clear the rubble away.

Meanwhile, more than 630 trucks transporting humanitarian supplies entered the Gaza Strip on Sunday, with at least 300 of them traveling to the territory's besieged and bombed north, said Tom Fletcher, the UN's under-secretary-general for humanitarian relief.

However, Fletcher also warned that there is still "no time to lose" in getting aid into Gaza. "After 15 months of relentless war, the humanitarian needs are staggering," he said on social media.

Agencies contributed to this story.