Ukrainian officials said two rebel-held towns had been recaptured and attempts were being made to take a village Kiev says was near the launch site of the surface-to-air missile that shot down the airliner with loss of all 298 on board.
Analysis of black box flight recorders from the airliner showed it was destroyed by shrapnel from a missile blast which caused a "massive explosive decompression", a Ukrainian official said on Monday.
Investigators in Britain, who downloaded the data, had no comment. They said they had passed information to the international crash investigation led by the Netherlands, whose nationals accounted for two-thirds of the victims.
In a report on three months of fighting between government forces and separatist rebels who have set up pro-Russian "republics" in the east, the United Nations said more than 1,100 people had been killed.
U.N. High Commissioner for Human Rights Navi Pillay said increasingly intense fighting in the Donetsk and Luhansk regions was extremely alarming and the shooting down of the Malaysian airliner on July 17 may amount to a war crime.
Western leaders say rebels almost certainly shot the airliner down by mistake with a Russian-supplied surface-to-air missile. Russia accuses Kiev of responsibility.
The separatists are still in control of the area where the plane was shot down but fighting in the surrounding countryside has been heavy as government forces try to drive them out.
On Monday at least three civilians were reported killed in overnight fighting, and Kiev said its troops recaptured Savur Mogila, a strategic piece of high ground about 30 km (20 miles) from where the Malaysia Airlines Boeing hit the ground, and other areas under rebel control.
A spokesman for Ukraine's Security Council, Andriy Lysenko, said Kiev was trying to close in on the crash site and force the rebels out of the area but was not conducting military operations in the immediate vicinity.
He said Ukrainian troops were in the towns of Torez and Shakhtarsk, both formerly held by the rebels, while fighting was in progress for the village of Snezhnoye - close to the presumed missile launch site - and Pervomaisk.
Government troops were also readying an assault on Gorlovka, a rebel stronghold north of the provincial capital Donetsk.
"The Ukrainian military is conducting an active assault on regions under temporary control of Russian mercenaries," Lysenko told a news conference in Kiev.
In Donetsk local officials said artillery fire had damaged residential blocks, houses, power lines and a gas pipeline. The city, with a pre-war population of nearly 1 million, has largely become a ghost town since rebels dug in for a stand in the face of advancing Ukrainian troops.
The site of the crash of the Malaysian airliner has yet to be secured or thoroughly investigated, more than 10 days after the crash. After days in which bodies lay untended in the sun, rebels gathered the human remains and shipped the bodies out, and turned over the flight recorders to a Malaysian delegation.
But the wreckage itself is still largely unguarded, and much of it has been moved or dismantled in what the rebels say was part of the operation to recover the bodies. No full forensic sweep has been conducted to ensure all human remains have been collected. Both side accuse the other of using fighting to prevent the investigation.
The Organisation of Security and Cooperation in Europe said its experts attempting to reach the crash site with investigators Australia and the Netherlands were forced to return to Donetsk for "security reasons".
A rebel leader, Vladimir Antyufeyev, told reporters in Donetsk that separatist fighters escorting the international experts to the site encountered fighting and turned back.
Antyufeyev, who like most of the senior rebel leadership is an outsider from Russia, also blamed the "senseless" Ukrainian army for trying to destroy evidence at the crash site under cover of fighting.
In Kiev, Australian Foreign Minister Julie Bishop, whose country lost 28 nationals in the crash, said she would discuss access with Ukrainian authorities.
"We'll be seeking assurances that any military action doesn't compromise our humanitarian mission," Bishop told a news conference. She hoped Russia would use its influence on the rebels to help allow wider access to the site.
Evidence could be lost if fighting continued, Australia's Deputy Commissioner of National Security, Andrew Colvin, said in Sydney, and the chances of finding the remains of all the dead grew slimmer as time passed.
European Union member states were expected to try to reach a final deal on Tuesday on further sanctions. The measures would include closing the bloc's capital markets to Russian state banks, an embargo on future arms sales and restrictions on energy technology and technology that could be used for defence.
The EU added new names on Friday to its list of individuals and companies facing travel bans and asset freezes over their alleged involvement in Ukraine and could agree to extend the list further as early as Monday.
Washington, which has taken the lead in imposing individual and corporate penalties on Russia, said on Friday it was likely to follow up on any new EU move with more sanctions of its own.
Russia said it would not impose tit-for-tat measures or "fall into hysterics" over Western sanctions, which could have the effect of make Russia more economically independent.
"We can't ignore it. But to fall into hysterics and respond to a blow with a blow is not worthy of a major country," Foreign Minister Sergei Lavrov said.
While Lavrov appeared to be trying to stake out the high ground amid growing tensions with the West, the EU was criticised for failing to stand up to Russia over Ukraine.
A prominent Polish newspaper editor and leading dissident during the Communist era, Adam Michnik, issued an open letter to EU leaders demanding fortitude in the face of what he called Russian President Vladimir Putin's "aggressive imperialism".
"Unity against Putin is the real answer to the crisis in Ukraine," Michnik said.