Egypt branded the Islamist gunmen responsible for the attack as "infidel" and promised on Monday to launch a crackdown following the massacre that strained Cairo's ties with both Israel and Palestinians.An Egyptian official has said "Jihadist elements" crossed from the Gaza Strip into Egypt before leading the assault on a border station. They then stole two armored vehicles and tried to smash their way into Israel, but were killed by Israeli fire. Egypt's military, which still holds many levers of power in the most populous Arab nation, called the attackers "infidels" and said it had been patient until now in the face of the instability in Sinai."But there is a red line and passing it is not acceptable. Egyptians will not wait for long to see a reaction to this event," it said in a statement on its Facebook page.Egypt closed its border crossing into Gaza overnight, cutting off the sole exit route for most Palestinians at the height of the Muslim-fast month of Ramadan.Hamas condemned the killings of the Egyptians and immediately sealed the warren of smuggling tunnels that connect Gaza with Egypt after Cairo said the gunmen had used these links to reach their territory.Many key goods, including oil, pass through the tunnels, and a prolonged closure could stymie life in the coastal enclave.Hamas said it was working with Egypt to try to identify those behind the bloody operation."No Palestinian could take part in such an ugly crime and in the killing of our beloved Egyptian army men in such a horrible manner," said Taher Al-Nono, spokesman of the Hamas Gaza government.Relentlessly hostile to Israel, Hamas is nonetheless considered overly moderate by many Salafi Islamists, who condemn Egypt's 1979 peace accord and demand a constant state of war with the Jewish state.Border penetrationLast August, eight Israelis died in a cross-border Sinai attack blamed on Palestinian militants from Gaza. In June, an Israeli worker died in another incident on the desert frontier.A Jihadist group called the "Magles Shoura al-Mujahddin" took responsibility for the June incident, saying it did so "without considering any claimed or imagined borders between Muslim countries. The Mujahddin has no word in their dictionary called borders except the boundaries of God."No one claimed immediate responsibility for Sunday's attack.On Sunday morning, an Israeli air strike killed a Palestinian gunman from a radical Islamist group and wounded another as they rode a motorbike in southern Gaza near the Egyptian border.Hours later, the group of unidentified gunmen ambushed the Egyptian border police - who had gathered to break the Ramadan fast after sunset - and opened fire, killing 16 security forces personnel and wounding at least seven.They then drove off in two vehicles. One of them exploded near the frontier while the second was hit by the air force after crossing the border and traveling some 2.5-km (2 miles) into the country, the army said.