Pro-Russian rebels in eastern Ukraine have expropriated ambulances to transport able-bodied fighters and threatened medical staff, according to a Human Rights Watch (HRW) report due out on Tuesday.

The organisation, which has also criticised pro-Kiev forces for using imprecise Grad rockets in civilian areas, found a number of instances of rebels stationing fighters at hospitals, seizing or destroying medical equipment and using ambulances to transport fighters.

"Pro-Russian insurgents' attacks on medical units and personnel are putting sick and vulnerable people and those who care for them at risk," said Yulia Gorbunova, HRW's Europe and Central Asia researcher. "This appalling disregard of people who are sick or wounded can be deadly and needs to stop immediately."

The organisation called on Moscow to use its leverage to prevent the abuse of medical equipment and facilities.

"With its influence over rebel forces in Ukraine, Russia should insist that they adhere to norms of international humanitarian law, including the special protections afforded to medical units and personnel as well as to the wounded and sick," the report said.

It also documented at least five cases of hospitals being hit by explosives, resulting in the deaths of at least two medical workers. HRW said it was not certain who had attacked the hospitals, but given that four of them were in areas controlled or occupied by insurgents, there was a strong suggestion that Ukrainian forces were responsible. It said even if rebel fighters had occupied hospital premises, it was not acceptable for Ukrainian forces to attack them there.

On Monday, Russia began major military exercises involving more than 100 aircraft near the border with Ukraine as the conflict intensified around Donetsk, a city of a million people held by the pro-Russian rebels who have been fighting Kiev government forces since April.

Heavy artillery fire continued there on Sunday night, according to the city administration. Ukraine's national security council said on Monday that government troops had begun "liberating Donetsk from the north after encircling the city".

The headquarters of Kiev's "anti-terrorist operation" said rebels had intensified artillery fire at its positions overnight.

A map published by the national security council showed government forces had made significant gains, cutting the rebels in Donetsk off from those in Lugansk. Ukrainian troops had taken back three-quarters of the territory formerly controlled by the rebels, the council said.

Ukraine's defence minister, Valery Geletei, said on Monday that Kiev would "be victorious very soon". Andrei Lysenko, a national security council spokesman, said the rebels would be defeated before the onset of winter.

The council also said that artillery based in Russia had fired on Ukrainian soldiers and border guards overnight. It was the latest in a string of claims by Ukraine and Russia accusing each other of cross-border fire.

The Russian military exercises that began yesterdayon Monday are scheduled to last until Friday and include Su-24, Su-27 and MiG-31 fighter jets, Mi-8, Mi-24 and Mi-28N helicopters, and the recently deployed Su-34 bomber. As part of the exercises, aircraft were to shoot at ground and air-based targets at newly opened testing grounds near Ukraine's south-east border, while air defence forces were to fire surface-to-air missiles near Astrakhan next to the Caspian Sea, the defence ministry said in a statement.

Fighter jets and bombers were also to practice mid-air refuelling from an Il-78 tanker plane above the Arctic Circle.