Ukraine's defence minister has said his forces are gaining ground significantly against rebels in the country's east.
Valeriy Heletey told the BBC there would be victory "very soon".
He blamed the rebels for the difficulties faced by international experts in getting access to the MH17 crash site.
His remarks came as civilians in the east prepare for a siege as government forces close in on the rebel-held cities of Donetsk and Luhansk.
Residents are stockpiling food and supplies and are sleeping in basements, with reports suggesting Luhansk is virtually surrounded and without power.
At least nine civilians were reported killed in Donetsk and Luhansk on Saturday.
The region has been unstable since April, when rebels in the east declared independence from Kiev. More than 1,500 people are believed to have been killed since fighting began.
The downing of Malaysia Airlines flight MH17 in July has heightened tensions and failed to stem the fighting.
Russia, which annexed Ukraine's Crimea region in March, has been accused of arming the rebels and has been targeted by US and EU sanctions. Russia denies the accusations.
'Russia is retaliating'
Col-Gen Heletey said he understood that it would not be easy to capture the cities of Donetsk and Luhansk, especially as Russia was "doing everything it can to provoke us".
But he said his forces were winning.
"The separatists' territory is now two-and-a-half times smaller than it was four weeks ago," he said.
"More than 65 towns and villages have been liberated by Ukrainian forces in the Donetsk and Luhansk region. Our forces are in an offensive phase, but I want the world to know, that Russia is retaliating."
The defence minister said he was 100% sure of victory and that Ukraine had no choice but to carry on fighting.
"Ukraine is like a burning house. The Ukrainian army is going inside the burning house, to put the fire out," he said.
"If we don't put it out, it will be in Kiev, in Kharkiv, everywhere. We are asking every Ukrainian: take a bucket of water and help us to put the fire out."
Col-Gen Heletey said that there would be no fighting on the MH17 crash site, currently controlled by the rebels, until all the evidence and remains had been collected.
"In practical terms, we are not far away from the crash site, but we have backed off from the area... so the international mission can be carried out properly," he said.
In the midst of the fighting, Dutch and Australian police are combing through the wreckage at the MH17 site for as many as 80 bodies thought to remain there.
Inspectors were forced to abandon their work on Saturday as shelling continued nearby.
The US has accused the pro-Russian rebels in the area of shooting down the airliner with a Russian-made surface-to-air missile, though Russia says Ukrainian government forces are responsible.