The head of NATO has called on Russia to "step back from the brink" of war by pulling its troops back from the Ukrainian border.

Anders Fogh Rasmussen, the US-led alliance's secretary general, warned further intervention in Ukraine would bring Russia greater isolation in the world.

Mr Rasmussen made his call during a visit to Ukraine in a show of solidarity after NATO warned of a possible invasion by Russia which, it said, had massed 20,000 troops near the frontier.

Russia's support for the rebels was growing in "scale and sophistication", he said.

"I call on Russia to step back from the brink, step back from the border and not use peacekeeping as an excuse for war-making," he added.

Earlier, he and Ukrainian Prime Minister Arseny Yatsenyuk discussed possible Western alliance support for Ukraine's defensive strength.

Kiev government forces continue losing men in clashes with pro-Russia separatists in the Russian-speaking east.

Rebels brought down a Ukrainian Mig-29 fighter plane with a missile this evening, Ukraine's military information spokesman Vladyslav Seleznyov told Interfax news agency.

The plane came down near Horlivka, about 100km from the border with Russia.

The plane's crew managed to eject from the aircraft, he said.

The Kiev government announced it was suspending a ceasefire with the rebels at the crash site of the Malaysian airliner that was shot down on 17 July, after an international mission halted recovery work there for security reasons.

Defence spokesman Andriy Lysenko told journalists that government forces had clashed 19 times with separatists in the 24 hours up to this morning, often coming under artillery and missile fire from within Russian territory.

"In the past 24 hours we have lost seven servicemen, and 19 received wounds," Mr Lysenko said.

Regional authorities in Donetsk, the east's main industrial hub and now the main rebel redoubt, said one person had been killed at a medical facility when a shell struck today after a night of intensive artillery attacks.

Government forces, who say they are gradually tightening a noose around the rebels, denied responsibility.

"We have accurate information that the Ukrainian military are not shelling those areas," said Mr Lysenko.

Meanwhile, the prime minister of Ukraine's self-proclaimed Donetsk People's Republic has announced that he is resigning his position.

Russian citizen Alexander Borodai said he was stepping down in favour of a local field commander, Alexander Zakharchenko, after finishing his work as a "start-upper".

Mr Borodai denied he was fleeing the unrecognised republic as a brutal government offensive closed in around the million-strong city.

"I think that the worst is over. Yes, we are in a very difficult military situation but every day this situation changes for the better and our armed forces grow stronger," he told journalists at a press conference in the rebel headquarters.

Putin's approval ratings on the rise

Russian President Vladimir Putin's domestic approval rating has soared to 87%, pollsters have said, which is the highest level in six years.

It comes as Russia is locked in a standoff with the West over Ukraine.

Nearly nine in ten Russians polled by the independent Levada research centre said they approved of Mr Putin's policies when the study was conducted at the beginning of August.

The figure is just one percentage point lower than the Kremlin strongman's historic popularity peak in August 2008 when Moscow fought a brief war with Georgia.

In the August poll, 66% of Russians said they think that the country is "moving in the right direction", against 19% who said Russia is on the wrong path.

Mr Putin's rating shot up from 65% at the start of 2014 after Russia annexed Crimea in March and his popularity remains strong as Russia continues its showdown with the West over the crisis in eastern Ukraine.

But his positive image at home contrasts sharply with how the Russian leader is viewed around the world.

A poll by Pew Research last month showed that Europeans and Americans view Russia more negatively than they did last year, with the percentage of Americans holding an unfavourable view of Russia up 29%.