The United States says Russia "continues to fuel the conflict" in Ukraine despite a cease-fire agreement signed in February in Minsk.
The U.S. ambassador to Ukraine, Geoffrey Pyatt, said during a July 28 briefing in Kyiv with journalists that Russia's actions in Ukraine have been "unpredictable and irresponsible."
He said: "I would underline the stakes for what is happening here. Our principles and shared trans-Atlantic values are at stake in the conflict under way today in Ukraine. The cardinal principle of respect for international borders and territorial integrity has been jeopardized. Ukraine is the front line of freedom in Europe."
Pyatt urged Russia and pro-Russian separatists in eastern Ukraine to implement the Minsk agreements, including the withdrawal of Russian troops and equipment from Ukrainian territory; the release of all "hostages," including Ukrainian pilot Nadia Savchenko; and the restoration of Ukrainian control over all its international borders.
Pyatt conducted the briefing together with the European Union's ambassador to Ukraine, Jan Tombinski.
Russia continues to deny that its military is involved in Ukraine's conflict.
Pyatt said developments on the ground in eastern Ukraine suggest "the Kremlin and its proxies are maintaining the capability to continue seeking to grab territory at a time and place of the Kremlin's choosing."
"Russian drones are operating over Ukrainian territory every single day," Pyatt said. "Russian-crewed surface-to-air missile systems are operating on Ukrainian territory."
He noted that Ukrainian forces on July 26 reported they had detained a Russian military officer driving a truck loaded with ammunition some 45 kilometers southwest of Donetsk.
The ambassador also rejected the notion of any quid pro quo with Moscow on Ukraine in exchange for Russia's recent support for a landmark international agreement to restrict Iran's nuclear program in exchange for the lifting of international sanctions against Tehran.
Pyatt said Russia supported the Iran agreement out of "its own self-interest."
"In Ukraine, we continue to have profound differences over Russia's continued violation of Ukraine's territorial integrity," Pyatt said. "And under no circumstances are we going to dilute those concerns in furtherance of some unrelated policy issue."
Pyatt also praised the Ukrainian government's reform efforts, describing them as "a second war" that Kyiv is fighting. He said the government has made progress on police reform, energy-sector reform, anticorruption measures, and military reform, despite having to simultaneously counter "Russian aggression" since the "invasion of Crimea" in the spring of 2014.