An Al-Qaeda inspired group claimed in a video it shot down an Egyptian military helicopter in the Sinai peninsula, in an escalation of a months-long insurgency that has spilled into the capital.
A video released by the Ansar Beit al-Maqdis group showed a surface to air missile hitting the helicopter, which burst into flames and plummeted to the ground.
The military has acknowledged five soldiers were killed in an incident involving a helicopter on Saturday, but said it was "an accident".
Military officials had said in the past they believed jihadists based in the lawless peninsula had acquired shoulder launched anti-aircraft missiles from neighbouring Libya.
Militants have killed scores of soldiers and policemen in attacks that have spread to Cairo following the army's overthrow of Islamist president Mohamed Morsi in July.
If the helicopter was indeed brought down by a missile, as witnesses have also said, it would present a threat not only to the military but also the country's crucial tourism industry.
"This is very significant," said David Barnett, an expert on Sinai jihadists with the US-based think tank the Foundation for Defence of Democracies.
"They could use these weapons to threaten the efficiency of military operations in the Sinai," he said.
Tourism could be impacted as well, he added, as hundreds of thousands of tourists take flights to south Sinai resorts on the Red Sea coast.
"Airlines are going to view this very seriously," he said.
Dutch charter airline Transavia had cancelled flights to the south Sinai resort of Sharm el-Sheikh in October because of the threat of such attacks.
"There is a threat from the ground in the Sinai area," Edmond Messchaert of the Dutch National Counterterrorism and Security Coordinator (NCTV) told AFP at the time.
Signup to our newsletter and follow us on Facebook and Twitter!
Weapons including shoulder fired missiles had flooded into Egypt following the Libyan uprising against Moamer Kadhafi in 2011, which opened up the former dictator’s weapon depots to rebels.
On Sunday, army chief Abdel Fattah al-Sisi was shown on state television attending a military funeral for the soldiers killed in the incident and consoling their families.
Barnett said the military appeared to be downplaying the incident because of its far-reaching consequences.
"From their perspective, because of the economic issue they have reason to downplay it," he said.
The video, which he said was authentic, also appeared to illustrate the military's difficulty in containing jihadist militancy in Sinai.
On Sunday, an ambush on a bus carrying troops in central Sinai killed four soldiers, security officials said.
The attack came after six bombs went off in the capital on Friday and Saturday, killing six people.
The deadliest was a car bombing outside Cairo police headquarters on Friday morning that killed four.
Ansar Beit al-Maqdis took responsibility for the attack.
The group is led by Islamist Bedouin in the peninsula, with help from extremists in the neighbouring Palestinian Gaza Strip, security officials have said.
It had claimed a number of brazen attacks outside Sinai before it bombed the Cairo security headquarters on Friday.
A suicide bomber with the group detonated a booby trapped car next to the interior ministers convoy in September. The interior minister was not hurt in the assassination attempt but a bystander died in the blast.
In December, it claimed responsibility for another suicide car bombing that struck a police building north of Cairo, killing 15 people.