* Attack takes place in former militant stronghold
* Al Qaeda branch leader believed in mountains
* Biggest attack since Amenas gas assault in 2013 (Updates death toll)
By Lamine Chikhi
ALGIERS, April 20 (Reuters) - Islamist militants killed at least 14 Algerian soldiers in an ambush in mountains east of the capital Algiers at the weekend in one of the deadliest attacks on the military in years.
The troops were in the Tizi Ouzou region, 120 km (75 miles) east of Algiers, when they were attacked by al Qaeda's north African branch, Al Qaeda in the Islamic Maghreb (AQIM), state news agency APS and a security source said on Sunday.
The attack on Saturday night came just days after President Abdelaziz Bouteflika, 77, was re-elected for a fourth term following a campaign that portrayed the ageing leader as the key to continued security for the North African OPEC state.
Since the end of its 1990s war with armed Islamists, attacks have been rarer in Algeria. But Algerian officials are concerned about spillover from the turmoil in neighbouring Libya, where fighters linked to al Qaeda take refuge in the southern deserts.
"On their way back from securing the presidential election, the unit came under attack," the defense ministry said in a statement posted by APS. Three militants were also killed.
Algerian security specialist Rahmani Anis told Reuters: "This attack is a response to setbacks for AQIM, which lost several of its militants in recent months. AQIM tried also to disrupt the election but it failed,"
AQIM is mostly based in the Sahel area which crosses southern Algeria. The army has killed 37 militants since January, according to the ministry, including several in the eastern mountains. Security sources say some have been found with weapons traced to Libya.
One of those believed to be hiding in the east is Abdelmalek Droukdel, a former chemistry student who become AQIM leader after fighting in Afghanistan.
Algerian forces last year killed two of Droukdel's deputies in Bouira, a former stronghold of militants during the 1990s civil war. That conflict was triggered in 1992 when the government cancelled elections that an Islamist party looked set to win.
In January last year, militants raided Algeria's Amenas gas plant near Libya's border, killing 40 oil contractors, most of them foreigners, in an attack that prompted British-based BP and Norway's Statoil to pull their workers out.
That attack was led by Mokhtar Belmokhtar, a veteran Algerian militant and former al Qaeda fighter whom the French have dubbed "The Uncatchable". He was reported killed last year though recent reports say he may still be alive. (Writing by Patrick Markey; Editing by Mark Trevelyan)