Armed forces have lost four Unmanned Aerial Vehicles within two years of its induction to boost border surveillance and Indian officials have taken up the matter with Israeli manufacturers, Defence Minister Pranab Mukherjee told Rajya Sabha on Thursday.

In-house probes into the crashes attributed them to system failure, Mukherjee said.

He said in a written reply that the original manufacturers had agreed to repair the UAVs free of cost.

While two of the UAVs that crashed were of the Indian Air Force, the other two belonged to the army.

In one of the accidents an IAF handler was also killed, officials said.

The armed forces top brass have expressed concern over the crash rate, which experts feel are unusually high for a total of 50 UAVs inducted so far.

The first crash of the UAV happened soon after induction in 2003 on a forward airbase in Rajasthan and the court of inquiry held human error as cause for the crash.

"A continuous and multi-faceted effort is underway in the defence forces to enhance and upgrade safety of the UAVs," Mukherjee said.

He added that measures to enhance the quality of training were also being pursued. The minister also said maintenance issues related to the UAVs had also been taken up with the Israeli manufacturers.

He said tactics had been formulated to avert aerial threat to UAVs from enemy aircraft during operational missions.

India entered into a major agreement with Israel for supply of High Altitude Searcher and medium altitude Heron UAV's in 2002, while DRDO also conducted final trials of the country's indigenous Nishant UAV.