Opposition leader Vitaly Klitschko said that Yanukovich had yielded nothing in a second round of talks with the opposition on Thursday evening, and he voiced fears that the impasse could now lead to further bloodshed.
At least three protesters were killed early on Wednesday in Kyiv - two from gunshot wounds - after clashes with riot police. Many of the protester have been led by a hard core of radicals.
Scores of others on both sides have been injured - many of them with eye injuries caused by flying projectiles and police rubber bullets.
After speaking first to protesters manning barricades at the main confrontation point with police, Klitschko then went to Kiev's Independence Square where he declared: "Hours of conversation were spent for nothing. There is no sense sitting at a negotiating table with someone who has already decided to deceive you.
"I earnestly wish that there will be no bloodshed and that people are not killed ... I will survive, but I am afraid there will be deaths, I am afraid of this," the boxer-turned-politician said.
Three opposition politicians - Klitschko, former economy minister Arseniy Yatsenyuk and far-right nationalist Oleh Tyahnybok - had tried to wring concessions from Yanukovich that would end two months of street protests against his rule.
Hundreds of thousands took to the streets in Kiev after Yanukovich backed away from signing a free trade deal with the European Union in favour of financial aid from Ukraine's old Soviet master Russia.
But the movement has since widened into broader protests against perceived misrule and corruption in the Yanukovich leadership.
Protesters have been enraged too by sweeping anti-protest legislation that was rammed through parliament last week by Yanukovich loyalists in the assembly.
Earlier on Thursday, Yanukovich had suggested he might be prepared to make concessions to the opposition when he called for a special session of parliament the next week to consider the opposition demands and find a way out of the crisis.
But this did not impress opposition leaders.
After several hours, the three opposition leaders emerged to say he had made no concessions at all and they ordered their followers to take immediate action to broaden the zone of protest in Kiev and in other cities.
"I believe we must go step by step - today a few towns, tomorrow there will be more. Today a few barricades - tomorrow more. We will extend the territory of the 'Maidan'," he said, using the local name for Kiev's Independence Square, the crucible of the protest movement.
Yatsenyuk, another opposition leader, said: "There was a list of demands that we did not get. Will we go back? No! So now we will build barricades," he declared.
Witnesses said that in response to the opposition call, about 1,000 demonstrators moved away from Independence Square and began to erect new barricades from sandbags packed with snow closer to the presidential headquarters.
Underlining the level of mistrust between the government and opposition earlier, Prime Minister Mykola Azarov accused protesters of trying to stage a coup d’état and dismissed the possibility of an early presidential election to resolve the standoff.
"All those who support this coup should say clearly, 'Yes, we are for the overthrow of the legitimate authorities in Ukraine', and not hide behind peaceful protesters," Azarov said at the World Economic Forum in Davos, Switzerland.
"A genuine attempt at a coup d’état is being carried out," Russian news agency Interfax quoted him as saying.
Azarov told Reuters that the government had no plans to introduce a state of emergency: "We don't see the need for tough and extreme measures at the moment ... But don't put the government into an impasse," he said.
"People should not think that the government lacks available resources to put an end to this. It is our constitutional right and obligation to restore order in the country."
Unrest has swollen in recent weeks and turned violent on Sunday when hard-core radicals broke away from the main protest area in the capital Kiev and clashed violently with riot police.
There are reports that protests are spreading to other provincial areas.
Thousands stormed the regional administration headquarters in Rivne in western Ukraine, breaking down doors and demanding the release of people detained in the unrest there, UNIAN news agency reported.
In the central Ukrainian town of Cherkasy, 200 km (125 miles) south of Kiev, about 1,000 protesters took over the first two floors of the main administration building and lit fires outside the building.
The turmoil has caused alarm abroad.
U.S. Vice President Joe Biden phoned Yanukovich on Thursday and warned him that failing to de-escalate the standoff could have "consequences," the White House said.
"The Vice President underscored that only the government of Ukraine can ensure a peaceful end to the crisis and further bloodshed would have consequences for Ukraine's relationship with the United States," the White House said.
German Chancellor Angela Merkel expressed anger over the crackdown.
"We are greatly worried, and not only worried, but also outraged at the way laws have been pushed through that call these freedoms into question," she told a news conference.
But Merkel added it would be wrong for Europe to respond to the violence with sanctions at this stage.
French President François Hollande called on Ukrainian authorities to "rapidly seek dialogue".
A European Commission spokesman said yesterday Yanukovich had spoken to President José Manuel Barroso. The spokesperson avoided speaking of sanctions, but Polish President Bronisław Komorowski said that international sanctions could be imposed on Ukraine at any moment unless the standoff between government and opposition ends.
"The Ukrainian authorities should be clear that we are on the brink of sanctions, that this is something that really could happen at any moment unless the spiral of conflict is halted," Poland's official PAP news agency quoted Komorowski as saying.
Former Soviet leader Mikhail Gorbachev urged the presidents of Russia and the United States to help broker negotiations, and said Ukraine was facing a possible "catastrophe".
Enlargement and Neighbourhood Commissioner Štefan Füle travels to Kyiv today in an attempt to help dialogue and calm tensions. Foreign affairs Chief Catherine Ashton is expected to fly to Kyiv next week.
In the meantime, a video showing inhumane treatment of an arrested protester stripped naked by Berkut special forces is making waves in social media and on international TV stations.
A chronology of the crisis is regularly updated in an evolutive article in English of the website Levyi bereg.