DONETSK, Ukraine (CNN) — Dutch and Australian experts trying to reach the crash site of Malaysia Airlines Flight 17 in war-torn eastern Ukraine turned back for the second day Monday after hearing explosions and being told of heavy fighting in the area.
The group -- including officials from the Netherlands, Australia and the Organization for Security and Co-operation in Europe -- returned to Donetsk, according to the Dutch-led investigation team.
Among other things, the team had hoped to work on the retrieval of human remains from the fields strewn with wreckage from the passenger jet, which had 298 people on board when it was brought down by a suspected surface-to-air missile on July 17.
The team of observers, investigators and experts had anticipated getting good access to the site after negotiating with both sides in the conflict, said Michael Bociurkiw, a spokesman for the Organization for Security and Co-operation in Europe mission, before the team was forced to turn back.
Ukrainian government forces have been battling pro-Russian rebels in the region for months, resulting in hundreds of deaths. Now, as Ukrainian troops attempt to cut off access to Donetsk, fighting is heading north, closer to the crash site, which sits amid rebel-held territory.
"This is a contested zone. There is active fighting going on," Andrew Colvin, deputy commissioner of the Australian Federal Police, said at a news conference Monday in Canberra.
Observers from the Organization for Security and Co-operation in Europe have visited the unsecured site several times since Flight 17 went down.
But the deputy mission head, Alexander Hug, told reporters the team stopped on the road to the crash site Sunday because they heard and saw the impact of artillery. He said it was unclear who was firing.
Malaysian deal with rebels
Colvin, the Australian police official, said a decision would be made with Dutch and the Organization for Security and Co-operation in Europe officials after an assessment of the fast-evolving situation on the ground.
"We are working on the basis that if it's a permissive environment, we will go in," he said.
The flare-up in fighting near the large crash site hampered the implementation of a deal that the Malaysian government struck with the rebels.
Rebel leader Alexander Borodai agreed to allow a deployment of international police officers to the site, Malaysian Prime Minister Najib Razak said Sunday.
"It is imperative that we deploy a full team of investigators to ensure that all the human remains are removed from the site, identified and repatriated," Najib said in a statement. "Everyone who was on board MH17 must be afforded proper dignity and respect"
Australian Prime Minister Tony Abbott had said he expected 49 officials, including 11 Australians, to visit the site as part of a Dutch-led team Sunday.
Of the 298 people killed in the crash, 193 were Dutch, 43 were Malaysian and 27 were Australian.
"Our objective is to get in, to get cracking and to get out," Abbott said at a news conference.
More than a dozen killed in clashes
But the deteriorating security situation has complicated matters.
In one example of the deadly clashes in the region, at least 13 people, including two children, were killed in fighting Sunday in and near the city of Horlivka, according to the Donetsk Regional Authority.
Horlivka is roughly 30 miles (about 50 kilometers) northwest of the MH17 crash site, but the fighting around the city is moving to the southeast.
Rebels are using Grad rockets on residential areas of Horlivka, according to the Ukraine's Counter-Terrorist Operations Press Center.
The Ukrainian military does not use artillery fire inside city centers to avoid civilian casualties, its statement said. The government said its forces were trying to encircle rebels in Horlivka and cut off their communication with Donetsk.
Responding to questions from CNN about the attacks in Horlivka, pro-Russian militant commander OE Khasanov said that the rebels are defending the city.
At least 332 Ukrainian soldiers have been killed and nearly 1,300 have been injured since April 7, according to Ukraine's National Defense and Security Council and law enforcement agencies. There is no known figure for the number of rebels killed.
Russia firing over border?
Debate continued over Russia's role in the conflict.
Russia has continued to increase its military presence near its border with Ukraine, the Ukrainian National Defense and Security Council media office said Sunday. It said two convoys were spotted moving toward a Russian town near southeastern Ukraine.
U.S. Secretary of State John Kerry has urged Russian Foreign Minister Sergey Lavrov to stop the flow of heavy weapons from Russia into Ukraine and start de-escalating the conflict, a senior State Department official told CNN on Sunday.
Kerry did not accept Lavrov's denial of allegations that heavy weapons from Russia were contributing to the conflict, the official said.
On Sunday, the U.S. State Department released satellite images from the Office of the Director of National Intelligence that it said show Russians firing into Ukraine.
Senior White House adviser Ben Rhodes said that the images confirm "what we have been saying publicly for several days."
"We are putting out evidence to confirm the points that not only has Russia been providing the weapons across the border, including heavy weapons," Rhodes said in an interview with CNN, "but they have also now been firing artillery barrages across the border."
Russia wants 'impartial' probe
At a news conference Monday, Lavrov said he hoped that deployment of an Organization for Security and Co-operation in Europe observer mission at two checkpoints on the Russian-Ukrainian border would clarify what was happening on the ground.
U.S. and Ukrainian officials have said a Russian missile system was used to shoot down Flight 17 from rebel territory. Russia and the rebels have disputed the allegations and blamed Ukraine for the crash.
Lavrov on Monday called for the investigation into the crash to be "impartial" and carried out with agreement from the U.N. Security Council.
Russia announced over the weekend it had formed a team to join the investigation, state-run news agency RIA Novosti reported. It will be led by the deputy head of the Federal Air Transport Agency, Oleg Storchevoy.
On July 16, the United States imposed its harshest sanctions yet on Russia over its annexation of Crimea, and what the Obama administration said was Russia's failure to end the Ukraine conflict.
Obama expanded sanctions on two banks, Gazprom Bank and VEB, and two energy companies, Novatek and Rosneft. The institutions will not be able to get important financing in the United States.
The administration banned American business contacts for eight Russian arms companies that make weapons, including small arms, mortars and surface-to-air missiles, including the Kalashnikov Concern, maker of the AK-47 assault rifle and other weapons.
Those sanctions have "driven down forecast of Russia's growth rate to near zero," Rhodes, the White House adviser, said Sunday.
But the measures have "not been enough to change" Russian President Vladimir Putin's "calculus" about "weapons going across the border."
"That is why we've said to Europe and our allies that we need to impose additional costs," Rhodes told CNN.
Europe, which imports nearly 30% of its energy from Russia, joined the United States in imposing limited sanctions earlier this year.
On Friday, the European Union said it was strengthening the sanctions despite concerns about the potential consequences.
Lavrov said Monday that the Russian government wasn't happy about the sanctions but that the country would overcome any economic difficulties and become more independent as a result.
"We are only seeing the desire of our Western partners to punish Russia," he said.
CNN's Nick Paton Walsh reported from Donetsk, Kyung Lah reported from Kiev, Ukraine and Jethro Mullen reported and wrote from Hong Kong. CNN's Susannah Palk, Mick Krever, Ashley Fantz, Ray Sanchez, Radina Gigova, Deborah Bloom and Victoria Butenko contributed to this report.