Hours after calling off a 10-day ceasefire Monday evening, Ukrainian forces launched a full-scale military operation against rebels in the eastern part of the country.

Announcing an end to the agreement, which many hoped would lead to peace talks, President Petro Poroshenko said in a televised address Monday night that Ukrainian government forces were launching a new "active phase" in what he termed an "anti-terrorist operation."

The first assaults reportedly hit the Donetsk and Lugansk regions during the president's address, where heavy fighting carried into Tuesday. According to journalist Christopher Miller, who is on the ground reporting for the Kyiv Post, there are indications of casualties in Kramatorsk, where a bus carrying civilians came under attack.

On Tuesday, Human Rights Watch issued a statement calling on Ukrainian government forces and armed insurgents to "respect the laws of war" and "distinguish at all times between combatants and civilians."

The truce was called off after discussions between the leaders of Ukraine, France, Germany and Russia on Monday failed to bring about a peaceful resolution.

In a speech Tuesday, Russian President Vladimir Putin vowed that he would continue to defend ethnic Russians using all means available from humanitarian assistance to "self-defense."

Lamenting the end of the truce, Putin continued:"Unfortunately, President Poroshenko has resolved to resume military action." He added that he and the European leaders "failed to convince him that the road to a secure, stable and inviolable peace cannot lie through war."

Putin's comments followed accusations by Russian Foreign Minister Sergei Lavrov, who over the weekend said that the United States was "pushing the Ukrainian leadership towards the path of confrontation."

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