At least 35 Egyptian troops and police officers were killed in clashes with Islamist fighters in the Bahariya oasis in the country's Western Desert on Friday, security and medical sources said.
An interior ministry statement confirmed the incident and said some of the attackers had died, without giving any figures for casualties or further details.
The small extremist group Hasm claimed the attack, saying in a statement that 28 members of the security forces were killed, with 32 injured.
Security forces, who are hunting down Islamic militants in the region, were ambushed late Friday on a road to the Bahariya oasis, some 200 kilometers southeast of Cairo, according to the interior ministry statement.
According to a source close to the security services, the convoy was hit by rocket fire. The attackers also used explosive devices.
Since the army removed President Mohamed Morsi, of the Muslim Brotherhood, extremist groups have increased their attacks on the country's military and police.
The Brotherhood, once Egypt's largest opposition movement, has long denied involvement in violence.
Mohamed Morsi was elected as Egypt's first civilian president in 2012, but the army overthrew him a year later following mass protests against the divisive Islamist's rule.
Since then, an extensive crackdown on the group has left it in disarray with competing wings that have disagreed on whether to use violence, after police quashed their protests.
Analysts say a section of the Brotherhood has encouraged armed assaults against policemen in Egypt.
Authorities have also been fighting the Egyptian branch of the jihadist group Islamic State, which has increased its attacks in the north of the Sinai peninsula.
Hundreds of soldiers and police have been killed in the violence.
The small group Hasm has claimed multiple attacks since 2016 on police, officials and judges in Cairo.
In their statements, none of the militant groups claim any affiliation to the Muslim Brotherhood.
The Islamic State group's deadly attacks on the military and police include a recent assault on a checkpoint in Sinai on July 7 that killed at least 21 soldiers.
The group has maintained a steady war of attrition with sniper attacks and roadside bombings.
But unlike their parent organisation in Iraq and Syria, they have been unable to seize population centres in the peninsula bordering Israel and Gaza.
In October 2015, IS claimed the bombing of a Russian airliner carrying holidaymakers from the popular South Sinai resort of Sharm el-Sheikh, killing all 224 people on board.