Donetsk, Ukraine (CNN) -- More death and violence were reported in eastern Ukraine on Sunday, as some investigators stood ready but were unable to go to the crash site of Malaysia Airlines Flight 17, downed more than a week ago.
At least 13 people, including two children, were killed in fighting on Sunday between pro-Russian rebels and Ukraine government forces in and near the Ukrainian city of Horlivka, according to the Donetsk Regional Authority, citing preliminary information from the local health care department.
Ukraine separatists are using Grad rockets on residential areas of Horlivka, according to Ukraine's Counter-Terrorist Operations Press Center in a statement.
Ukraine's military does not use artillery fire inside city centers to avoid civilian casualties, the statement said. Horlivka is surrounded by government forces.
Pro-Russian militant commander OE Khasanov responded to CNN's Nick Paton Walsh's questions about the attacks in Horlivka by saying that the separatists 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 separatists killed.
Crash site remains 'still under control of terrorists'
Sunday's bloodshed comes more than a week after the Malaysia Airlines Flight went down July 17 in eastern Ukraine. All 298 on board were killed.
Though some investigators from the Organization for Security and Co-operation in Europe have been at the scene, Malaysian investigators haven't been able to access the entire crash site, officials said Sunday.
Malaysia has struck an agreement with pro-Russian separatists in the area to allow international police to protect investigators, Malaysian Prime Minister Najib Razak said Sunday, through a statement from his office. But he said he is "deeply concerned" those investigators still are not on scene due to the "volatile security situation."
He implored that "the human remains are removed from the site, identified and repatriated.
"Everyone who was on board MH17 must be afforded proper dignity and respect," he said. Forty-three of the people on MH17 were Malaysian, including two infants and 15 crew members.
The crash site remained Sunday "still under control of terrorists," Andriy Lysenko, the spokesman for Ukraine's National Security and Defense Council told reporters.
The OSCE deputy mission head, Alexander Hug, told reporters that the team stopped on the road to the crash site because they heard and saw the impact of artillery. He said it was unclear who was firing.
The OSCE has been in contact with the separatists and the government to take another try at reaching the site Monday, he said.
Hug said rebels have offered a train to transport bodies that the investigators find at the scene.
Dutch investigators -- among them a team of 30 forensic experts -- are remaining for the moment in the city of Donetsk, which is about 75 kilometers (50 miles) from Hrabove. Some 40 Dutch police officers are also positioned in Donetsk for now.
About two-thirds of the people who died in the plane crash, 193 of the 298, were from the Netherlands.
Russian team ready to join probe
Russia announced 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.
Russia has continued to increase its military presence near its border with Ukraine, the National Defense and Security Council press office said earlier Sunday. It said two convoys were spotted moving toward a Russian town near southeastern Ukraine.
Ukrainian forces are trying to encircle militants in Horlivka and cut off their communication with Donetsk, the press office said, hours before news emerged Sunday of victims killed in fighting.
U.S. Secretary of State John Kerry has urged Russian Foreign Minister Sergey Lavrov to stop the flow of heavy weapons and rocket and artillery fire from Russia into Ukraine and to start de-escalating the conflict, a senior State Department official told CNN on Sunday.
Kerry did not accept Lavrov's denial 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, in an interview with CNN, 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," said Rhodes, "but they have also now been firing artillery barrages across the border."
The separatists are "Russian proxies," he said.
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, Novotek 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 and other weapons.
Those sanctions have "driven down forecast of Russia's growth rate to near zero," Rhodes said Sunday.
But the measures have "not been enough to change" 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.
Earlier this year, Europe, which imports nearly 30% of its energy from Russia, joined the United States in imposing limited sanctions.
CNN's Nick Paton Walsh in Donetsk, journalist Victoria Butenko in Kiev and CNN's Ray Sanchez and Radina Gigova contributed to this report. CNN's Ashley Fantz wrote this story in Atlanta.