* Largest loss for Egypt forces in decades
* Police patrol ambushed in remote western desert
* Islamic State battling army in northern Sinai (Adds details throughout)
By Ahmed Mohamed Hassan and Ahmed Tolba
CAIRO, Oct 21 (Reuters) - Egypt's security forces suffered one of their heaviest attacks after militants firing rockets and detonating explosives hit a police operation on Friday in the western desert, authorities and security sources said on Saturday.
Three security sources said at least 52 police officers and conscripts had been killed in gun battles that erupted in the remote desert, but Egyptian authorities said only 16 men had died when their patrol came under attack.
Egyptian authorities on Saturday said two police operations were moving in on a suspected militant hideout on Friday when one of the patrols came under fire from heavy weapons in a remote area around 135 km southwest of Cairo.
The interior ministry said 16 police were killed in that part of the operation, and 13 more were wounded. At least 15 militants were also killed in the gun fight. The statement did not give details on any casualties in the other police patrol.
"As soon as the first mission approached the location of the terrorist elements, they sensed the arrival of the forces and targeted them using heavy weapons from all directions," the interior ministry said in a statement.
One security source said the convoy was attacked from higher ground by militants firing rocket-propelled grenades and detonating explosive devices on the ground.
Security has been a key point for President Abdel Fattah al-Sisi, a former military commander who presents himself as a bulwark against terrorism after leading the ouster of president Mohamed Mursi of the Muslim Brotherhood in 2013. Sisi was elected on a landslide a year later.
Though he has yet to declare his intentions, Sisi is widely expected to run for re-election in April next year with little opposition. But critics say his popularity has been dimmed because of security and economic austerity policies.
No group made any claim or statement about Friday's operation not far from the capital. But most of the fighting so far between militants and security forces has been in northern Sinai, where an Islamic State affiliate operates.
Security sources earlier said the police had been hunting hideout of the Hasm Movement, an Islamist militant group blamed for attacks on judges and police around the capital.
That group has in the past only carried out mostly small operations since it emerged last year. Egyptian authorities say it is the armed militant wing of the Muslim Brotherhood, an Islamist group it outlawed in 2013.
Most of its leadership has been jailed in a crackdown under Sisi. Since Sisi came to power, hundreds of troops and police have been killed in often sophisticated attacks by militants in the northern Sinai region, where Ansar Bayt al-Maqdis group pledged allegiance to Islamic State in 2014.
Last Sunday, at least 24 militants and six soldiers were killed in attacks on military outposts in North Sinai, when more than 100 militants repeatedly attacked security outposts south of the border town of Sheikh Zuweid.
Attacks have mostly hit police and armed forces, but militants have also extended their campaign outside the Sinai, targeting Egypt's Christians with bomb attacks on churches in Cairo and other cities.
(Writing by Patrick Markey; Editing by Stephen Powell)
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