Militants shot dead five Egyptian conscripts in the Sinai Peninsula on Thursday, as the army searched for eight servicemen missing after an attack on a navy boat in the Mediterranean.
The military carried out air strikes in Sinai, killing three members of the Islamist militant group Ansar Beit al-Maqdis, security officials said.
Egypt has been hit by a wave of attacks since the army overthrew Islamist president Mohamed Morsi last year, infuriating his supporters.
In Cairo, 16 people were injured Thursday in a panicked crush on a Cairo metro train after a small bomb exploded during rush hour.
The attacks in Sinai, in which two police conscripts and three soldiers were taken out of their vehicles and shot dead, bore the hallmarks of Ansar Beit al-Maqdis, which often sets up impromptu checkpoints in the lawless peninsula.
The ambushes came a day after an assault on a navy boat wounded five servicemen and left eight lost at sea.
The military, which said late Wednesday it was still conducting search and rescue operations for those missing, has called the incident a "terrorist" attack.
But the identities and goals of the assailants remained unknown a day later.
A security official said that dozens of suspects rounded up at sea after the assault were still being interrogated.
It was not immediately clear whether they were "terrorists" or drug and weapons smugglers who use that part of the sea, he said.
Four boats used by the assailants were destroyed, according to the military.
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State-owned Ahram newspaper reported that a naval patrol approached three boats which aroused suspicion and it came under fire, citing an anonymous source saying they were most likely smugglers.
It added that a protest later erupted in a Damietta village after reports that the military arrested fishermen at sea following the exchange of fire.
-Militants intercept conscripts-
The government is fighting an Islamist militant insurgency that has killed scores of policemen and soldiers, but such a maritime attack was a first.
The incident came days after Ansar Beit al-Maqdis pledged its allegiance to the Islamic State jihadist group in Iraq and Syria.
Egypt's army has launched an unprecedented crackdown in Sinai to quell the militants, razing homes along the border with Gaza to create a buffer zone with the Islamist-controlled Palestinian enclave.
One of the attacks Thursday took place at the entrance of the town of Rafah along the border with Gaza, where the army is demolishing homes to create the buffer zone.
The other occurred several kilometres (miles) to the west, in the north Sinai town of Sheikh Zuweid.
Ansar Beit al-Maqdis has spearheaded an insurgency in the peninsula that has killed scores of policemen and soldiers since the army overthrew Morsi.
It is believed to have been behind an attack on a military checkpoint last month that killed at least 30 soldiers -- the deadliest such incident in years.
In a statement on Thursday, the military said it had arrested a member of Ansar Beit al-Maqdis's decision-making council along with 25 suspected militants.