* Monitors call off crash site visit due to fighting
* Malaysia says separatists agree to allow investigators access
* Ukrainian government says troops advancing to crash site (Adds details on fighting)
By Aleksandar Vasovic
DONETSK, Ukraine, July 27 (Reuters) - Fighting around the site of the Malaysian airliner downed in Ukraine prevented a visit by international experts on Sunday, although Malaysia said separatists had agreed to allow in international police and investigators.
The Ukrainian government said its forces were advancing towards the crash site to try to free it from the rebels, who have impeded the work of international monitors and whom Kiev accuses of tampering with evidence pointing to who shot it down.
"All our troops are aiming to get there and liberate this territory so that we can guarantee that international experts can carry out a 100-percent investigation of the site and get all proof needed to deduct the real reason of this tragedy," said Andriy Lysenko, a spokesman for Ukraine's Security Council.
International monitors said the fighting itself could affect the crash site, underlining the growing complexity of trying to establish who shot down the plane. Western leaders say it appears to have been downed in error by separatists supplied by Moscow; Russia has suggested Ukrainian forces are to blame.
In Donetsk, Alexander Hug, deputy head for the Organization for Security and Cooperation in Europe's monitoring mission in Ukraine, said monitors would not visit the site on Sunday.
"The situation on the ground appears to be unsafe ... we therefore decided to deploy tomorrow morning," Hug, flanked by Dutch and Australian experts, told reporters.
"Fighting in the area will most likely affect (the) crash site," Hug said.
The separatists are still in control of the area where Malaysia Airlines flight MH17 was shot down earlier this month but fighting in the wider eastern regions of Donetsk and Luhansk has been heavy as Ukrainian government forces try to dislodge them. It was raging in at least five different places on Sunday.
Lysenko said government troops were advancing east from the town of Makievka towards another town of Shakhtarsk, nearby the crash site. Shakhtarsk residents said air strikes hit the city.
"Our military is advancing, fighting goes on every day, every night, they have already liberated two-thirds of the territory," held told a news conference in Kiev.
Earlier, Malaysian Prime Minister Najib Razak said an agreement reached with separatist leader Aleksander Borodai would "provide protection for international crash investigators" to recover human remains and ascertain the cause of the crash.
The OSCE has provided a team to monitor the site in advance of an investigation, but Najib said a full team of investigators was needed to ensure any human remains left there were removed.
"We also need a full deployment of investigators to have unfettered access to the crash site so we can understand precisely what happened to MH17. I hope that this agreement with Mr Borodai will ensure security on the ground, so the international investigators can conduct their work," he said.
"Three grieving nations", referring to Malaysia, Australia and the Netherlands, had formed a police group to secure the site, he said in a statement issued by his office.
Among the 298 people who died aboard the Boeing 777 on its flight from Amsterdam to Kuala Lumpur on July 17 were 193 Dutch nationals, 43 Malaysians and 28 Australians.
TIME TO PROCEED
Najib on Saturday said the separatists had fulfilled two of three conditions of a earlier deal struck - the return of victims' bodies and of the plane's two "black boxes" - and it was now time to proceed with the investigation.
Malaysian experts have said they believe at least 30 investigators will be required to cover the full site of the crash, in addition to Dutch investigators and an expert from the United Nations' civil aviation body, the ICAO.
In the Australian capital Canberra, Prime Minister Tony Abbott said an unarmed police mission led by the Netherlands and made up of about 49 officers would travel to the site. Officials said a total of 170 Australian police were deployed in Ukraine.
"Our objective is to get in, to get cracking and to get out," Abbott, who has played a leading role in pressing for an investigation, told reporters in Canberra.
"This is a risky mission, no doubt about that, but all the professional advice I have is that the safest way to conduct it is unarmed, as part of a police-led, humanitarian mission."
Abbott said the force, including 11 Australians, would stay "as long as we can to do a thorough job" but he expected it would be no longer than three weeks.
Officials said the separatists had become more cooperative and Abbott said this reflected a general change in attitude.
"I think what's kicked in in recent days is our common humanity - our common feeling that this is dreadful beyond words," he said.
The Dutch Safety Board said last week it had taken control of an investigation into the crash and would coordinate a team of investigators from Ukraine, Malaysia, Germany, the United States, Britain, Russia and the ICAO.
(Additional reporting by Natalia Zinets and Lina Kushch in Kiev, Yantoultra Ngui in Kuala Lumpur and Morag Mackinnon in Perth; writing by Gabriela Baczynska and Elizabeth Piper, editing by Philippa Fletcher)