Heavily-populated western provinces of Aleppo, Deraa, Homs, Idlib and Rural Damascus are most affected

BEIRUT, May 12 (Reuters) - About 5.1 million Syrians are living in areas at high risk from explosive weapons, some of which fail to detonate and so will pose a deadly threat for years to come, aid group Handicap International said.

The global charity examined 78,000 violent incidents in Syria's war between December 2012 and March 2015 and found that more than 80 percent involved highly destructive weapons like rockets, mortars and bombs, rather than light arms.

"Syria will inherit the deadly legacy of explosive weapons for years," said Anne Garella, Regional Coordinator of Handicap International.

The conflict is in its fifth year, having killed more than 220,000 people, according to a U.N. estimate. More than 1.5 million people have been injured, according to the Syrian Observatory for Human Rights, an organisation that monitors the war.

Diplomatic efforts aimed at finding a political solution have gone nowhere.

A range of insurgent groups have been battling the Syrian military and allied fighters. U.S.-led air forces have been bombing ultra hardline Islamic State militants since last summer.

The Handicap International study found that three-quarters of the incidents it recorded took place in populated areas like larger towns and cities.

"This suggests that belligerents have no intention of effectively distinguishing between civilians and combatants -- which constitutes a violation of international humanitarian law," said the group, which provides aid to disabled people in conflict and disaster zones.

The heavily-populated western provinces of Aleppo, Deraa, Homs, Idlib and Rural Damascus were the most affected, the study found. The research was based news reports, social media and data from the United Nations and non-governmental organisations.

(Reporting by Sylvia Westall; editing by Andrew Roche)

Our Standards: The Thomson Reuters Trust Principles.