Ukraine's president has announced plans for a unilateral ceasefire as part of a plan to end fighting in the east of the country. The announcement came not long after Petro Poroshenko spoke by telephone to his Russian counterpart, Vladimir Putin, raising hopes that the latest attempt to solve the crisis might win the support of the Kremlin. But separatist forces in east Ukraine were dismissive of the idea, and promised to keep fighting.

"The plan will begin with my order for a unilateral ceasefire," said Poroshenko, speaking at a military academy in Kiev. "I can say that the period of the ceasefire will be rather short. We anticipate that immediately after this, the disarming of the illegal military formations will take place."

Mikhail Koval, Ukraine's defence minister, said the ceasefire could begin in days.

Poroshenko spoke to Putin late on Tuesday, and the conversation "touched on the theme of a possible ceasefire", the Kremlin said. The Ukrainian president also discussed the plan with Angela Merkel, the German chancellor.

The Russian foreign minister, Sergei Lavrov, said that the ceasefire should be permanent. He did, however, say that if negotiations followed it could be "the step President Poroshenko has promised and which in general we were all waiting for".

Rebels in the self-declared Donetsk People's Republic reacted to Poroshenko's proposal with scorn and disbelief. Many suspected the move was a ruse to lull them into complacency.

Mikhail Verin, commander of the Russian Orthodox army, one of the most powerful militias in Donetsk, said Poroshenko was "deceiving Russia and the European Union" and playing for time to reposition his forces for further attacks on the rebels.

Alexander Khodakovsky, commander of the most powerful rebel fighting group, the Vostok Battalion, said the Ukrainian army had already violated a temporary ceasefire agreed on Tuesday by firing mortars on Wednesday afternoon at rebel positions near Snezhnoye.

He argued that Poroshenko "can't control all the forces in the anti-terrorist operation", which includes many recently recruited national guard forces and irregular militias fighting alongside regular army troops. The Kiev government also did not have the political will to actually make peace in the east, he said.

"Poroshenko is forced to take up weapons and continue the war but at the same time make unclear statements about a one-sided ceasefire," Khodakovsky said.

"He's giving us an ultimatum to disarm, but what if people don't lay down their weapons? Closing the border amid the current conditions here is pushing people into a corner," he added.

If there was a ceasefire, said militia commander Roman Ivlev, all Kiev forces must withdraw from the Donetsk People's Republic and neighbouring Lugansk People's Republic before a peace process could start. Ivlev leads the Donbass Veterans' Union, a rebel group comprising more than 1,000 former servicemen, police officers and members of the Berkut riot police, the shock troops of former president Viktor Yanukovich who fought with protesters in Kiev this winter.

Speaking before a Russian Orthodox prayer service for his fighters outside their headquarters, Ivlev said that authorities' promises of peace were "used to bring in reinforcements and take out their wounded". He struck a note of defiance, saying that the rebels also did not believe Poroshenko's promises of amnesty for those who voluntarily disarmed. "We won't put down our arms, because as soon as we do they will come and take us with naked hands," he said. "How can the head of a foreign government amnesty us?"

"After the events in Odessa, Ukraine died here," he added, referring to the clashes between pro-Russian and pro-Ukrainian protesters in that coastal city that ended in a fire that killed dozens. "Ukraine won't exist here ever again."

Ivlev, a former police officer and leader of the local Cossack military caste, said his ultimate goal was the conquest of the rest of Ukraine, the "gathering of Russian lands" and the creation of a Slavic-led Eurasian union to rival the European Union.

Rank-and-file rebels also reacted to the proposed ceasefire with scepticism. Two militiamen standing guard outside the rebel-occupied Donetsk administration building, who would identify themselves only by their wartime nicknames "Picasso" and "Suicide Bomber", said they did not believe Poroshenko.

"I won't put my arms down until they leave here," Picasso, a Russian army veteran with a Kalashnikov rifle, said.

"I gave my oath to the Soviet Union. That's the only government that can demand anything of me," said Suicide Bomber, a Cossack.

Fighting has continued in the east, with two Russian journalists the latest casualties as the Ukrainian army shelled separatist fighters on Tuesday. More than 300 people are believed to have died since the fighting began.

Kiev has blamed Russia for arming and directing the rebels in the east, and Poroshenko said on Wednesday that Ukraine was caught up in a "new type of war". "This is war … using professional diversionary groups, mercenaries, volunteers and the local population. The volunteers and the local populations have been brainwashed by the information war."

The office of the UN high commissioner for human rights released a major report on Ukraine on Wednesday. The high commissioner, Navi Pillay, said the armed separatists in the east should put down their arms, accusing them of drawing themselves and the local population into a "dead end, which is leading simply to misery, destruction, displacement and economic deprivation".

She also told the Ukrainian government to "exercise restraint, ensure that security operations are in line with international standards at all times, and take great care to protect civilians when engaging with armed groups".

Russia's investigative committee said it was investigating Ukraine's interior minister and a billionaire regional governor on suspicion of criminal acts connected with the military operation in the east.

Vladimir Markin, spokesman for the committee, said in a statement that Arsen Avakov and Ihor Kolomoisky were "literally drowning the country in the blood of its own people" and said they were being investigated on a range of charges. It was unclear how a Russian body had the jurisdiction to investigate the alleged crimes.

Poroshenko also announced that he plans to replace a number of key ministers, including the foreign minister.

Andriy Deshchytsia, the current acting minister, achieved notoriety over the weekend after being caught on camera calling Putin a "dickhead". Russia's foreign ministry said it would never speak to Deshchytsia again after the incident.

Joe Biden, the US vice-president, told Poroshenko by phone on Wednesday that the United States would work with its allies to "impose further costs on Russia" if Moscow failed to use its influence to stop the separatist violence and kept sending weapons and militants across the border, the White House said.