EL-ARISH, Egypt -- Egypt's military vowed on Monday to hunt down those behind the killing of its 16 soldiers at a checkpoint along the Sinai border with Israel. It called the attackers "enemies of the nation" who must be dealt with by force and suggested they were Egyptian Sinai-based militants who received Palestinian support from the Gaza Strip.
Security and military officials said at least two helicopter gunships arrived in the border town of El-Arish on Monday to join the hunt for the militants believed responsible. Israel meanwhile stepped up pressure on Egypt to clamp down on the lawless border region.
Israel says its aircraft killed eight militants who broke through the border after the killings. Egyptian officials have said six attackers were killed. A statement by the Egyptian armed forces said 35 militants took part in the attack, suggesting that close to 30 attackers may be on the run.
No group has yet claimed responsibility for the attack. Egypt and Israel say both Islamist militants from the Sinai and Palestinian allies from the neighboring Gaza Strip are active in northern Sinai, attacking both Egyptian security forces and staging raids across the border into Israel.
The armed forces statement suggested that groups on both sides of the border may have been involved.
"The armed forces have been careful in the past months and during the events of the (Egyptian) revolution (in 2011) not to shed Egyptian blood ... but the group that staged yesterday's attack is considered by the armed forces as enemies of the nation who must be dealt with by force," said the statement.
In the first direct indication that the attackers may have had the help of Palestinian militants, the statement said "elements from the Gaza Strip" aided the attackers by shelling the Egyptian-Israeli border crossing of Karam Abu Salem with mortars as the attack was taking place.
The security and military officials said counter-terrorism units arrived in the border town of El-Arish on Monday and joint police-army patrols were combing the ground. Aircraft patrolled the sky overhead, they said.
The officials said the two attack helicopters were expected to be joined by more aircraft in the border zone, which has seen a surge of violence since Hosni Mubarak's ouster last year.
The Sunday attack was one of the deadliest in the Sinai in years. Suspected Islamists attacked the checkpoint in the border town of Rafah at sunset, killing the soldiers as they were sitting down for the traditional meal breaking the fast in the holy month of Ramadan.
The attackers then commandeered two of their vehicles and burst through a security fence into Israel. Israeli officials say the incursion was quickly spotted and hit with an airstrike. The Egyptian military said only one armored vehicle was commandeered.
The unrest in Sinai poses a daunting challenge to Egypt's Islamist President Mohammed Morsi, who since coming to office a little more than a month ago has warmed up to Gaza's Hamas rulers. Hamas officials have condemned the killings, but Morsi may still come under pressure to back down from plans to end Egypt's cooperation with the Israeli blockade of the Gaza Strip.
He vowed on Sunday night to make the killers pay for their crime and to restore security to Sinai, home to several of the most popular Red Sea resorts in Egypt. On Monday, he declared three days of mourning for the victims, according to state television.
"This is a huge calamity for Egypt," declared his spokesman Yasser Ali.
The Sinai border has been largely quiet for most of the three decades since Israel and Egypt signed a peace agreement, although security forces have for years combated a low-level insurgency in El-Arish and nearby areas. The 1979 treaty restricts the number of troops and the type of weapons Egypt can deploy in the peninsula.
Sinai has experienced a security vacuum since Mubarak's overthrow in February 2011, and both cross-border and other insurgent attacks have increased. Israel has agreed in the past to Egypt sending reinforcements to bolster its forces there, but the Egyptian officials did not say whether Israel had signed off on the helicopter deployment.
The officials spoke on condition of anonymity because they were not authorized to speak to the media.
Israeli Prime Minister Benjamin Netanyahu on Monday paid an unannounced visit to the site where the militants broke through, praising Israeli security forces for their swift response and expressing regret for the loss of the Egyptian lives.
"Israel and Egypt have a shared interest in maintaining a quiet border," Netanyahu said. "But when talking about the security of Israeli citizens, Israel must and will rely only on itself," he added.
Other Israeli officials gave more details of the attack and their response.
Chief military spokesman Brig. Gen. Yoav Mordechai told Army Radio that after killing the soldiers, the militants seized a truck and an armored car, then blew up one of the vehicles to punch a hole through a security fence to enter Israel.
He said that Israeli intelligence services had reports of impending infiltration and sent aircraft to strike as the militants broke through. "We were prepared for it, so there was a hit," he said. The military "averted a major attack on southern Israel," he added.
Defense Minister Ehud Barak told parliament that eight militants were killed by Israeli forces who struck from the air, as well as with tanks and artillery.
The attack was the third cross-border infiltration since Mubarak's overthrow. In one, in August 2011, eight Israelis and six Egyptian soldiers were killed. The six Egyptians were mistakenly killed by Israeli forces. Israel is building a fence along the border to block militants as well as illegal African migrants, but also wants Egypt to crack down harder on the border region.
"We hope this will be a fitting wakeup call for the Egyptians to take things in hand on their side more forcefully," Barak told parliament's foreign affairs and defense committee on Monday.
Egypt-Israel relations have always been cool but since Mubarak was overthrown and Islamists rose to power, Israeli officials have expressed concern that ties would further deteriorate. Israel is particularly concerned that Egypt will ease restrictions on entering and leaving the Gaza Strip.
Immediately after the attack, Egypt shut the Rafah crossing with Gaza, an ominous sign for the Palestinian territory's 1.6 million people. Israel bars Gazans from entering Israel, so the Rafah crossing -- the only exit from the tiny coastal territory not under direct Israeli control -- is their sole gateway to the outside world.
Gaza officials disowned the attack but gave mixed signals over whether residents of the territory may have carried it out.
Gaza's deputy prime minister, Mohammed Awad of the ruling Hamas movement, said militants from the territory were "not involved in this awful crime." But a leading Hamas member, Mohammed Zahar, undercut that denial, telling Al-Jazeera TV that he asked Egypt to provide the names of possible suspects from Gaza so that "we will immediately bring them to justice."
The attack could harm Hamas' efforts to persuade Morsi to ease restrictions at the crossing. Hamas is an offshoot of the Muslim Brotherhood and had hoped that Morsi, a fellow member of the region-wide movement, would be sympathetic to its requests. But he has moved cautiously, in part because of concerns about an influx of militants from Gaza.
Teibel reported from Jerusalem. Associated Press reporters Ibrahim Barzak in Gaza City, Gaza Strip, and Mohammed Daraghmeh in Ramallah, West Bank, contributed to this report.