Ukrainian armed forces mounted a major onslaught against pro-Russian separatist fighters Sunday in an attempt to gain control over the area where a Malaysia Airlines plane was downed earlier this month.

The U.S. State Department, meanwhile, released satellite images that it says back up its claims that rockets have been fired from Russia into eastern Ukraine and heavy artillery for separatists has also crossed the border.

A four-page document released by the State Department seems to show blast marks from where rockets were launched and craters where they landed. Officials said the images, sourced from the U.S. Director of National Intelligence, show heavy weapons fired between July 21 and July 26 — after the July 17 downing of Malaysia Airlines Flight 17.

Moscow has angrily denied allegations of Russia's involvement in eastern Ukraine. Russia's foreign ministry over the weekend accused the U.S. of conducting "an unrelenting campaign of slander against Russia, ever more relying on open lies."

Secretary of State John Kerry spoke by phone Sunday with Russian Foreign Minister Sergei Lavrov, urging him to stop the flow of heavy weapons and rocket and artillery fire from Russia into Ukraine, said a State Department official. Kerry did not accept Lavrov's denial that heavy weapons from Russia were contributing to the conflict, said the official, who spoke on condition of anonymity to provide details of the call.

There was no immediate comment from Moscow.

Reports of the intensifying unrest in eastern Ukraine prompted a postponement of a trip to the site by a team of Dutch and Australian police officers who had planned to start searching for evidence and the remaining bodies.

In the Netherlands, Prime Minister Mark Rutte said his government has rejected the idea of deploying armed troops to secure the crash site because there is no way they could achieve "military superiority" in a region where heavily armed pro-Russian rebels are battling Ukrainian government forces.

"The option we looked at was a military option in which you could secure the area so you can work in a stable environment," Rutte said. But "that the option would be such a provocation to the separatists that it could destabilize the situation."

Ukraine's National Security Council said Sunday that government troops have encircled Horlivka, a key rebel stronghold, and that there had been fighting in other cities in the east. Horlivka lies around 20 miles (30 kilometers) north of the main rebel-held city of Donetsk.

The armed forces "have increased assaults on territory held by pro-Russian mercenaries, destroyed checkpoints and positions and moved very close to Horlivka," the council said in a statement.

A representative of the separatist military command in Donetsk confirmed that there had been fighting in Horlivka, but said that rebel fighters were holding their positions.

Elsewhere, Russian state news agency RIA Novosti reported Sunday that a column of Ukrainian armored personnel carriers, trucks and tanks had entered the town of Shakhtarsk, 10 miles (15 kilometers) west of the site of the Boeing 777 crash.

Shakhtarsk is a strategic town in the area. By controlling the town, the Ukrainian army would cut off vital rebel supply lines.

Local media reported fighting also taking place in the towns of Snizhne and Torez, the two nearest mid-sized towns to the crash site.

The government accused rebel forces of firing rockets Sunday on residential apartment blocks in Horlivka in what they said was an attempt to discredit the army and whip up anti-government sentiment. The separatist self-declared "Donetsk People's Republic" has accused the army of being responsible for that and other rocket attacks in nearby cities.

The Donetsk regional government — which is loyal to Kiev and based elsewhere since rebels took over the area — said Sunday in a statement that at least 13 people, including two children aged 1 and 5, were killed in fighting in Horlivka. It said another five people were killed as a result of clashes in a suburb north of Donetsk.

New York-based Human Rights Watch last week condemned what it said was the Ukrainian government forces' practice of using unguided rockets in populated urban areas. It said that use of the rockets was a violation of international humanitarian law that "may amount to war crimes."

Malaysian Airlines Flight 17 was shot down with a surface-to-air missile over a part of eastern Ukraine controlled by pro-Russian separatists on July 17, killing all 298 people on board. U.S. and Ukrainian officials say it was shot down by a missile from rebel territory, most likely by mistake.

Ten days after the disaster, a full-fledged investigation still has not begun at the crash site, with some bodies still unrecovered and the site forensically compromised. Concerns about the integrity of the site were raised further when the parents of a young woman who died in the disaster that had flown from their home in Perth, Australia, visited the site Saturday outside the village of Hrabove and even sat on part of the plane's wreckage.

It remained unclear when the forensic experts from the Netherlands and Australia would be able to begin their work at the site.

Alexander Hug, the deputy head of a monitoring team from the Organization for Security and Cooperation in Europe, said it was too dangerous for the unarmed officers to travel there from their current location in Donetsk.

"We reassess the situation continuously and we will start to redeploy tomorrow morning back to the site if the situation changes," Hug said.

Australian Prime Minister Tony Abbott had said earlier Sunday that unarmed Australian police would be part of the Dutch-led police force to secure the area and help recover victims' remains.

Abbott said that by using unarmed police, Ukraine's parliament won't need to ratify the deployment as it would if the security force were to be armed.

"This is a risky mission. There's no doubt about that," Abbott told reporters. "But all the professional advice that I have is that the safest way to conduct it is unarmed, as part of a police-led humanitarian mission," he said.

Malaysian Prime Minister Najib Razak said in a statement that his country would send dozens of police and that his country had received assurances from pro-Russia separatists that they would provide protection for investigators.

Flights from Ukraine to the Netherlands have taken 227 coffins containing victims of the plane disaster. Officials say the exact number of people held in the coffins still needs to be determined by forensic experts in the Netherlands.

The Malaysia Airlines disaster prompted some expectations in the West that Russia would scale back its involvement in the uprising in Ukraine's east, but the opposite seems to be the case.

In addition to producing evidence that rockets have been fired into Ukraine from Russia, the U.S. has said it has seen powerful rocket systems moving closer to the border.

Separately, the New York Times reported Sunday that U.S. defense and intelligence officials were working on a plan that would enable the Obama administration to give Ukraine specific locations of surface-to-air missiles controlled by Russian-backed separatists in eastern Ukraine. The plan, if implemented, would allow the Ukraine government to target these missile sites for destruction, the newspaper said.

Citing U.S. sources, the Times said it was unclear if President Barack Obama would want to give Ukraine the more precise information about military targets because it would amount to America getting more involved in the conflict.

In Warsaw, Poland, about 250 people marched through the city to protest what they called the "terror" imposed by Russian President Vladimir Putin in Ukraine. Some of the demonstrators carried Ukrainian flags, and there were banners that proclaimed "Putin is a Sponsor of Terror" and "Europe, Stop Just Talking. Start Taking Action! Stop Terror in Ukraine."