15 bodies have been recovered from the site of a rocket strike on a refugee convoy in Ukraine, a military spokesman has said.

The convoys were understood to have been evacuating civilians from the towns of Svitlivka and Khrashchuvate which lie along the main road leading from besieged Luhansk and the Russian border.

"By 7pm last night we retrieved 15 bodies. The search continued into the night and is continuing today," said spokesman Andriy Lysenko.

Ukrainian government forces have blamed the attack on pro-Russian separatists; the rebels have denied responsibility.

The convoy had been in an area of fierce fighting between government forces and rebels when it came under fire from rebel Grad and mortar launchers, according to a Ukraine spokesman.

"The force of the blow on the convoy was so strong that people were burned alive in the vehicles - they weren't able to get themselves out," military spokesman Anatoly Proshin told Ukrainian news channel 112.ua.

The rebel leader of the self-styled Donetsk People's Republic, Alexander Zakharchenko, said yesterday: "Not a single convoy of refugees was shot at in the Lugansk region."

United Nations Secretary-General Ban Ki-moon was said to be deeply disturbed by the attack.

"He urges all sides to respect the rules governing the status of internally displaced persons and refugees and to allow safe passage to anyone attempting to leave areas of active military operations," said his spokesperson, Stéphane Dujarric.

The road where the attack occurred is likely to be used by a convoy of Russian aid if Ukraine allows it to enter the country.

The International Red Cross - which will take responsibility for the aid convoy inside Ukraine - has demanded security guarantees from all sides but this has yet to be agreed.

More than 2,100 people have been killed in four months of conflict inside Ukraine. The UN estimates more than 285,000 people have fled fighting in the east.

Fierce fighting continued around Luhansk over the weekend.

Meanwhile, Russia is said to be working on further retaliatory measures in the event Western nations implement fresh sanctions over the Ukraine crisis, according to President Putin's spokesman Dmitry Peskov.