The U.S. State Department has released satellite images that Washington says support claims rockets have been fired from Russia into Ukraine during the last week.
The State Department released a four-page document on July 27 that seems to show blast marks from where rockets were launched and where they landed.
Officials said the images show heavy weapons being fired between July 21 and 26.
Russia's Foreign Ministry said Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry spoke by telephone and agreed on the need for an early cease-fire in eastern Ukraine.
Moscow described the Ukrainian crisis as an "internal conflict."
The U.S. State Department reported that Kerry rejected Lavrov's denial that part of the problem lay in heavy weapons from Russia. In their phone conversation, Kerry urged Lavrov "to stop the flow of heavy weapons and rocket and artillery fire from Russia into Ukraine, and to begin to contribute to deescalating the conflict," according to the State Department, via Reuters.
The U.S. ambassador to Kyiv, Geoffrey Pyatt, earlier tweeted the satellite images that Washington said clearly showed shelling by artillery from Russian territory and directed at Ukrainian troops.
Meanwhile, at least 13 people, including two small children, were killed in the eastern Ukrainian city of Horlivka on July 27.
Officials from the Donetsk regional administration said only that the deaths on July 27 were the result of "military actions in Horlivka," without mentioning which side might have been responsible.
Ukrainian forces launched an offensive against pro-Russian separatists in the Donetsk region on July 26.
Fighting has been reported around Horlivka, once a city of some 250,000 people.
The fighting caused a team of international inspectors to give up for the day plans to visit the crash site of MH17.
The setback came hours after Malaysia announced it had agreed with Ukrainian rebels to allow international police personnel to enter the crash site.
Alexander Hug, deputy head of the Organization for Security and Cooperation in Europe's (OSCE) monitoring mission in Ukraine, told reporters in Donetsk that "we heard indications there's fighting going on."
He added, "The situation on the ground appears to be unsafe...we therefore decided to deploy tomorrow morning."