Qatari-bought fuel arrived at the Gaza Strip's only power station after entering through Israel, sources said, in a bid to alleviate conditions in the besieged enclave and stem any escalation in Israeli-Palestinian violence.
A Palestinian source at the Kerem Shalom crossing in southern Gaza said six trucks monitored by the United Nations and carrying 450,000 litres of fuel crossed on Tuesday.
"The Qatari fuel to the Gaza Strip's power plant today is aimed at partially improving electricity (supply) in Gaza," Hamas spokesman Hazem Qassem told AFP news agency.
The delivery could help ease months of protests and clashes along the border between Israel and Hamas-run Gaza, which has been under a crippling Israeli and Egyptian blockade for more than a decade.
The blockade has deprived the strip's roughly two million inhabitants of several vital commodities, including food, fuel and medicine.
The trucks that entered Gaza brought the first delivery out of a $60m fuel donation by Qatar meant to provide the power plant with enough fuel to operate for six months, local sources said.
For months, Gazans have been receiving only four hours of mains electricity a day on average.
A spokesperson for the Palestinian Prime Minister Rami Hamdallah, who is based in the Israeli-occupied West Bank, voiced disapproval of the fuel delivery.
"Any international financial aid to the Gaza Strip should be through, or with the coordination of, the Palestinian government," he said, in order "to preserve Palestinian unity" and to stop any plans to separate Gaza from the West Bank.
In a statement on Tuesday Azzam al-Ahmad, a senior official close to Palestinian President Mahmoud Abbas, threatened retaliatory measures if the fuel deliveries continued.
The United Nations has warned that Israel's 11-year blockade of the strip has resulted in a "catastrophic" humanitarian situation.
Under the UN-brokered deal, Qatar pays for the fuel which is then delivered through Israel with United Nations monitoring, a diplomatic source said.
A Qatari official, speaking to Reuters news agency on Sunday, said Doha planned to help with Gaza's power crisis "at the request of donor states in the UN, to prevent an escalation of the existing humanitarian disaster".
Israel's energy minister, Yuval Steinitz, told Reuters on Monday that Qatar "was trying to help" prevent a Gaza flare-up.
Hamas seized control of Gaza from Abbas's Palestinian Authority in a 2007 civil war.
Multiple reconciliation attempts aimed at restoring the PA to power in Gaza have failed.
Hamas spokesperson Qassem said that the deliveries were facilitated "through the United Nations because of the vacuum left by the PA".
Abbas says that making deals with Hamas amounts to recognising their control over Gaza in place of the PA.
Much of the international community considers Hamas a terrorist organisation. It has rejected calls for it to disarm and recognise Israel.
Israel says its blockade of Gaza is necessary to isolate Hamas, with which it has fought three wars since 2008.