The US military has delivered three radars to Ukraine designed to detect incoming mortar fire, the Pentagon said Friday, amid appeals from Kiev for Washington 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, Pentagon spokesman Colonel Steven Warren said.

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.

"It will be up to the Ukrainians how, when and where they deploy these systems," Warren said.

President Barack Obama has so far ruled out providing weapons and ammunition to Ukraine, and instead approved the delivery of "non-lethal" assistance such as radars, night vision goggles, radios, rations, body armor and other items.

But at a Senate Foreign Relations Committee hearing earlier this week, Tony Blinken, who is nominated for a senior diplomatic post, said the United States should consider providing Ukraine with "defensive" weapons.

"I think it is something that we should be looking at," Blinken said.

Ukrainian leaders and some US lawmakers have repeatedly urged Obama to send arms to the Kiev government but the Pentagon said there had been no change in the current approach.

"To my knowledge, there is no new policy decision to announce," Warren told reporters.

Russia, which denies Western accusations it is supplying and advising rebels in eastern Ukraine, has warned the Americans against arming Ukrainian government troops.