At least 21 people,<b> </b>including seven children, were killed in a fire in the<a href="https://www.thenationalnews.com/tags/gaza-strip/" target="_blank"> Gaza Strip</a>, the Palestinian official news agency reported on Thursday. An investigation has been opened into the cause of the blaze, a civil defence spokesman told Palestinian news outlets. Hamas militants, who control the Israeli-blockaded Palestinian enclave, said firefighters had contained the blaze in Jabalia that left charred walls and mounds of black soot. The head of the Indonesian Hospital in Jabalia, Saleh Abu Laila, told AFP that the centre had received the bodies of at least seven children. Initial reports indicate the fire was caused by large amounts of benzene stored in an apartment. Footage posted to social media showed the entire building, in the Tar Al Zatar area of the camp, ablaze as large crowds gathered at the site. Palestinian President Mahmoud Abbas, in the Israeli-occupied West Bank, considered the fire to be "a national tragedy", his spokesman said. Mr Abbas declared a day of mourning on Friday, with flags to be flown at half-mast, and offered to send aid to families of the victims to "ease their suffering", spokesman Nabil Abu Rudeineh said. Witnesses told Reuters they were unable to help people trapped inside because of the intensity of the blaze. Senior PA official Hussein Al Sheikh urged Israel to open the Erez crossing that connects Gaza with southern Israel and is normally closed at night. This would allow critically hurt patients to be moved "to treat them outside the Gaza Strip if necessary", Mr Al Sheikh said. Israeli Defence Minister Benny Gantz tweeted that his staff would assist with "humanitarian evacuations of the injured to [Israeli] hospitals", expressing sympathy over the "serious disaster". A large crowd of onlookers gathered on the street outside the multi-storey home as the blaze raged, sending plumes of smoke billowing out the top of the concrete building. Jabalia is a refugee camp is in the northern Gaza Strip and is the largest of eight refugee camps in the densely populated enclave. The camp now includes large buildings and in many respects resembles a city. The UN Works and Relief Agency, which helps Palestinian refugees, says the camp is overcrowded and many residents live in substandard conditions. Crowds remained on the street, with hundreds of police and emergency response workers on hand, after the blaze had been extinguished. Gaza, densely populated with 2.3 million people, has been under Israeli blockade since 2007, a measure Israel says is necessary to contain threats from armed groups in the strip. With electricity supply sparse in the impoverished territory, domestic blazes are common, as Gazans seek alternative sources for cooking and light, including kerosene lamps. This year Gaza received an average of 12 hours of mains electricity daily, up from just seven hours five years ago, according to UN data. New dangers arise in the winter when many people burn coal for heat. Hamas said an investigation was under way to determine the cause.