As the violence in eastern Ukraine intensifies, Human Rights Watch reports that Russian-backed rebel forces have detained, assaulted and executed civilians.
The rebels have gained ground in recent weeks, including taking Novoazovsk, a town near the strategic port of Mariupol.
Yesterday NATO released satellite images showing Russian troops entering eastern Ukraine, despite Russian denial of arming and supporting the Ukrainian rebels. It is estimated that there are now more than 1,000 Russian troops on Ukrainian soil.
Around 2,600 people have been killed since April, according to United Nations figures.
In August, Human Rights Watch researchers in Ukraine interviewed civilians who had been beaten, kicked, stabbed, burned with cigarettes, and some who and been subjected to mock executions.
At least six were used as hostages, either for ransom or in exchange for captured insurgents held by the Ukrainian authorities.
Executions without trial and civilian deaths in custody are also cause for concern. Human Rights Watch came into possession of three death sentences against civilians apparently issued by the insurgents' summary war tribunal. Two were marked 'executed'.
"Pro-Russian insurgents are regularly committing horrendous crimes," said Hugh Williamson, Europe and Central Asia director at Human Rights Watch. "There are solid grounds to be seriously concerned about the safety and well-being of anyone held by insurgent forces in eastern Ukraine."
Ukrainian President Petro Poroshenko cancelled a visit to Turkey on Thursday in response to the "rapidly deteriorating situation" in the eastern region, "as Russian troops have actually been brought into Ukraine".
He described the situation as "extraordinarily difficult but controllable". The President tried to calm Ukrainians, saying: "Destabilisation of the situation and panic: this is as much a weapon of the enemy as tanks."
UN secretary general Ban Ki-Moon said he was concerned by the "dangerous escalation" of the crisis, as an emergency UN Security Council meeting was held yesterday to discuss the situation.
At the meeting, US ambassador to the UN Samantha Power said: "Russia has come before this Council to say everything except the truth. It has manipulated. It has obfuscated. It has outright lied.
"So we have learned to measure Russia by its actions and not by its words. In the last 48 hours, Russia's actions have spoken volumes."
Russian ambassador Vitaly Churkin responded by saying: "Everybody knows that there are Russian volunteers in eastern Ukraine."
Chances of a foreign military response are slim, however, as Ukraine is not a member of NATO.
President Barack Obama said yesterday: "It is important to recognise that a military solution to this problem is not going to be forthcoming."
Obama avoided describing Russian intervention as an 'invasion'. "I consider the actions that we've seen last week a continuation of what's been taking place for months now," he said.