DONETSK, Ukraine (AP) -- 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.