Ukrainian security officials on Friday said Russian forces have tightened their control on territory in eastern Ukraine after crossing the border to support separatist rebels.
Meanwhile, a new United Nations report documents serious human rights abuses committed by both sides in the conflict.
Ukrainian national security spokesman Andriy Lysenko told reporters in Kyiv Friday that Russian troops are collaborating with the rebels to strengthen their hold on Novoazovsk, a key Ukrainian town near the border with Russia.
Russian forces are patrolling its streets with members of the main separatist authority, the Donetsk People's Republic, checking the registration of civilian cars and detaining those who are registered elsewhere, Lysenko said.
"Districts are being mined," he said, "while Russian military shelling has damaged a bread factory and a local agricultural plant."
Ukrainian officials have accused Russia of escalating its involvement in the months-long conflict by sending soldiers and equipment across the border and increasing artillery attacks from Russian soil.
NATO on Thursday released satellite images purportedly documenting the movement of Russian supplies in Ukraine.
Russia repeats denials
Russia continues to deny allegations it is supporting the rebels, calling the conflict a Ukrainian domestic issue.
In an address to separatist fighters published Friday, Russian President Vladimir Putin said Ukrainian military operations in the east represent a "grave danger" to the civilian population.
Citing a "humanitarian catastrophe," he called on Kyiv to immediately stop its military actions.
He also called on pro-Moscow separatists to free Ukrainian soldiers that have been surrounded in eastern Ukraine. In a Kremlin statement Friday, he urged the insurgents to "open a humanitarian corridor" for the troops in order to avoid "senseless deaths."
Rebel leader Alexander Zakharchenko told Russian media that he is ready to comply with the request, but that the soldiers must first give up their weapons.
UN shares data
The United Nations said Friday that nearly 2,600 people have been killed since fighting broke out in mid-April. That does not include the 298 people killed when a Malaysian airliner was shot down July 17 above the war-torn area.
Fighting has intensified in August, with roughly three dozen people killed daily in eastern Ukraine, according to the U.N.'s assistant secretary general for human rights.
A pro-Ukrainian fighter is held in a garage in the eastern Ukraine town of Novoazovsk Aug. 29, 2014.
Ivan Simonovic, speaking at the report’s release in Kyiv Friday, expressed particular concern about increased Russian involvement.
"Especially disturbing is the alleged increase in participation of foreign fighters in hostilities, which further fuels the conflict," Simonovic said. The leader of an armed group "stated yesterday that thousands of Russian citizens, including regular army soldiers on leave, are fighting beside them."
Violations attributed to both sides
The U.N. report says armed rebel groups have been responsible for most of the abuses taking place in the east, including abductions, physical and psychological torture and forced recruitment.
Battalions operating under Ukrainian government control are also accused of abuses such as unlawful detention, forced disappearances and torture.
Simonovic called for efforts to ensure that perpetrators of human-rights violations are brought to justice.
"People, while they are in the midst of conflict, tend to forget that the moment of truth will come and that the moment of individual responsibility will follow," Simonovic said.
Ukraine's international allies, as well as the United Nations and NATO, have expressed concerns about reports of increased Russian military involvement in the conflict.
On Thursday, U.S. President Barack Obama accused Russia of deliberately violating Ukraine's sovereignty and warned that Moscow could face further economic sanctions.