A top NATO commander has said that columns of Russian tanks and other military equipment have been identified entering Ukraine.

U.S. General Philip Breedlove, Commander, U.S. European Command, as well as the Supreme Allied Commander Europe of NATO, referred to "Russian tanks, Russian artillery, Russian air defense systems and Russian combat troops," during a trip to Bulgaria on Wednesday. It is unclear how NATO observed the Russian movements, although the military bloc released satellite images of Russian movements along the border in the past.

This is has led to speculation that Ukrainian forces were preparing for all-out war. 

NATO’s claim coincides with an Organization for Security and Co-operation in Europe (OSCE) report of 43 unmarked vehicles traveling through rebel-held areas this week. That convoy included five towed artillery guns and five “multi-launch” rocket systems, according to AFP. OSCE observers saw “More than 40 trucks and tankers” moving within rebel territory, half of which towed large artillery pieces.

Russian defense ministry official General-Major Igor Konashenkov told the Russian Interfax news agency that “there was and is no evidence” of Russian movement in Ukraine, according to Reuters.

Russia has been suspected by NATO, the U.N. and the West of being behind much of the violence in Ukraine since the annexation of Crimea in March and the beginning of conflict in East Ukraine in April, a charge Moscow has consistently denied. However, pictures coming out of the conflict zone show Russian flags on tanks and Russian military insignia on troops. 

Pro-Russian separatist rebels have said that "volunteers" from Russia have helped in their efforts.

The incursion by Russian troops comes as the city of Donetsk was rocked by heavy artillery fire on Wednesday morning. According to a BBC report, it was unclear whether the fire came from Ukrainian government forces or the rebels, or both. Donetsk, the heavily contested “capital” of the self-proclaimed Donetsk People’s Republic, has become ground zero for the conflict and tens of thousands of residents have fled over the last eight months.

In Luhansk, another rebel-held city, one Ukrainian soldier was killed and another injured as rebels fired on government positions near the village of Schastya, according to Ukrainian security forces. 

More than 4,000 people have died since the beginning of the conflict in spring of 2014. A ceasefire was signed in Minsk on Sept. 5, but has largely not been observed as hundreds more lost their lives. 

Angela Merkel, Chancellor of Germany, said on Tuesday that the Minsk agreement had not been followed, but at present there were no plans to increase sanctions against Russia. 

"Further economic sanctions are not planned at the moment, we are focusing on the winter and the humanitarian situation there and how to get a real ceasefire," she said.

The Organization for Security and Cooperation in Europe (OSCE) said there was a major risk that the conflict could worsen. 

"The level of violence in eastern Ukraine and the risk of further escalation remain high and are rising," OSCE representative Michael Bociurkiw told reporters in Kiev. "More than two months after the signing of the Minsk protocol memorandum, the firing has not ceased."

A Russian funeral service reportedly transported bodies from the Donbas region to Russia through a rebel-controlled border point overnight this week, according to Colonel Andriy Lysenko, spokesman for the National Security and Defense Council of Ukraine. Lysenko said those bodies could be those of Russian Federation soldiers who fought in Ukraine.

Around 4,000 people have died in the eight-month-long conflict.