Ukraine said its troops had taken more territory from pro-Russian rebels near the site where Malaysia Airlines flight MH17 was brought down. International investigators said fighting had again prevented them reaching the crash location.
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.
"The Ukrainians have taken over a part of the crash site," said Vladimir Antyufeyev, the self-styled first deputy prime minister of the Donetsk People's Republic.
Ukrainian officials said two rebel-held towns had been recaptured and attempts were being made to take a village Kiev says is near the launch site of the surface-to-air missile that shot down the airliner with the loss of all 298 on board.
Watch: Fighting stops international team reaching Ukraine crash site
UN High Commissioner for Human Rights Navi Pillay said increasingly intense fighting in the Donetsk and Luhansk regions was extremely alarming and that the shooting down of the Malaysian plane on July 17 may amount to a war crime.
The site of the crash of the Malaysian plane has yet to be secured or thoroughly investigated, more than 10 days after the crash.
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.
Russian foreign minister Sergey Lavrov blamed Ukrainian military action, saying that a buffer zone announced by Ukraine President Petro Poroshenko had not been honored.
"The reality is Ukrainian authorities have to stop their fighting and respect the resolution of the UN Security Council and provide full access to the crash site," Lavrov said, adding that Russia wanted investigators to "find out the truth".
"The first priority is that the investigation will be impartial," he said.
The Organisation for Security and Cooperation in Europe said its monitors attempting to reach the crash site with investigators from Australia and the Netherlands were forced to return to Donetsk for "security reasons".
Antyufeyev, the rebel leader, said 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 from Russia, also blamed the 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 said.