The US military has delivered three radars to Ukraine designed to detect incoming mortar fire, amid appeals from Kiev to send weapons to help fight pro-Russian rebels.

The counter-mortar radar systems were flown to Ukraine in a C-17 cargo plane that accompanied US Vice President Joe Biden, who paid a visit to Kiev on the first anniversary of protests that unleashed a year of upheaval.

A total of 20 counter-mortar radar systems were due to be delivered over the next several weeks, and Ukrainian troops would undergo training on the radars starting in mid-December.

The radars detect incoming mortar rounds and then calculate the origin of the mortar fire.

The systems can be hooked up to mortar or artillery batteries which then return fire.