The Obama administration later said it had placed 20 top Ukrainian officials on a visa blacklist.
European diplomats and senior officials in Brussels began working on the logistics of imposing sanctions, in preparation for an emergency meeting of European Union foreign ministers called for Thursday.
They also announced that the French, German and Polish foreign ministers would visit Kiev beforehand, to meet with members of the government and the opposition.
President Obama warned the Ukrainian military to stay out of the crisis.
“We have been watching very carefully and we expect the Ukrainian government to show restraint, to not resort to violence in dealing with peaceful protesters,” Mr. Obama said during a one-day visit to Mexico to meet with Mexican and Canadian counterparts.
“There will be consequences if people step over the line.”
In Washington, the State Department said it had imposed a visa ban on 20 senior Ukrainian officials, whom it accused of playing a role in the government’s crackdown.
The State Department declined to say which officials were on the blacklist, which prevents them from traveling to the United States, but a senior State Department official said, “the list today includes the full chain of command responsible for ordering the violence last night.”
The official, who spoke on condition of anonymity, said that if the violence continued, the United States, working with the European Union, would impose sanctions against Ukrainian officials “in a much broader and deeper way.”
While this official said there were no signs of military involvement in the crackdown on Tuesday night, American officials were closely monitoring a “changing of the guard” at military barracks on Wednesday, saying it raised concerns that troops could be yet mobilized.
Earlier Wednesday Secretary of State John Kerry, who was visiting Paris for meetings on the Middle East, had also warned that the United States might join a European sanctions response to the Ukraine crisis.
“We are talking about the possibility of sanctions or other steps with our friends in Europe and elsewhere,” Mr. Kerry said in a joint appearance in Paris with the French foreign minister, Laurent Fabius.
President François Hollande of France, speaking at a joint news conference with Chancellor Angela Merkel of Germany after a regularly scheduled meeting in Paris, said, “There are unspeakable, unacceptable, intolerable acts being carried out in Ukraine.”
Ms. Merkel, condemning the “shocking pictures which are reaching us from Ukraine,” made plain that sanctions would be applied against those responsible for the violence.
E.U. foreign ministers have only to decide “which specific sanctions should be applied,” she said.
“But sanctions alone are not enough,” she added.
“We have to get the political process going again,” including both government and opposition representatives.
She added that the French and German foreign ministers and top officials were using every available channel, including to Russia, to defuse the crisis.
Mr. Kerry said the purpose of his warning to Ukraine was to “create the environment for compromise,” and that United States did not think it was too late for Ukraine’s president, Viktor F. Yanukovych, to negotiate with the opposition.
“President Yanukovych has the opportunity to make a choice,” Mr. Kerry said.
“Our desire is for Mr. Yanukovych to bring people together, dialogue with the opposition, find the measure of compromise and put the broad interests of the people of Ukraine out front.”
Obama administration officials have said in recent weeks that the United States was prepared to move ahead with sanctions if Mr. Yanukovych cracked down on the opposition.
American officials signaled on Wednesday morning that sanctions were under consideration.
Poland’s foreign minister, Radoslaw Sikorski, said in a telephone interview en route to Kiev that “we will impose some sanctions tomorrow in Brussels.”
But Mr. Sikorski said it was unclear whether sanctions would be imposed on Mr. Yanukovych himself.
“After all, he’s still the democratically elected president, large numbers voted for him and he still controls the situation, though he’s made a complete mess of it,” Mr. Sikorski said.
Prime Minister Donald Tusk of Poland said earlier that he would make the case for immediate measures against those who provoked the escalation that has left dozens dead and hundreds wounded.
“We Poles will not remain indifferent to these events, because we know that the developments in Ukraine will decide the history and the future of the whole region, and thus also influences the future and the security of Poland,” said Mr. Tusk, who held an emergency cabinet meeting late Tuesday as events unfurled in Kiev.
The imposition of so-called smart sanctions is a standard foreign policy reflex of the European Union when confronted by acts of violence.
Over the years, officials have honed a system for identifying individuals deemed responsible for repression, usually imposing a travel ban preventing visits to the 28-nation European Union and often freezing bank accounts.
Experts debate their effectiveness when applied to countries like Zimbabwe, but their likely use in this case underlines the extent to which the bloc’s efforts to use its influence in its own neighborhood have faltered.
Nevertheless, they appear highly likely to be introduced, José Manuel Barroso, the president of the European Commission, the executive arm of the 28-nation bloc, having also said he expects targeted measures against those responsible for violence.
Elmar Brok, chairman of the European Parliament’s Committee on Foreign Affairs, called for sanctions against specific individuals responsible for the violence.
“This is not the time for news releases,” he told a news conference in Brussels.
“We don’t call for sanctions against Ukraine but against certain responsible people.”
Mr. Brok suggested a travel ban on leadership figures and measures to investigate the bank accounts held by individuals in countries such as Germany, the Netherlands and Britain.
He was speaking alongside Ruslana Lyzhychko, a Ukrainian pop singer and protest leader, who said she would go on hunger strike if such sanctions were not introduced.
If officials and diplomats decided on Wednesday and Thursday to go ahead with sanctions against individuals, they would be agreed formally by European foreign ministers at their emergency meeting Thursday.
Any list of those targeted would normally be made public only when published in the European Union’s Official Journal, which puts sanctions into legal effect.
The earliest that is likely would be Friday.
The system is designed to avoid giving warning to those whose bank accounts are about to be frozen.
The German foreign minister, Frank-Walter Steinmeier, also issued a warning to the Ukrainian government.
“Whoever is responsible for the decisions which have led to the bloodshed in Kiev and other parts of Ukraine should expect Europe to reconsider its position on imposing sanctions on individuals,” Mr. Steinmeier said in a statement, as lethal mayhem engulfed the center of Kiev Tuesday night.
Hours earlier, Mr. Steinmeier telephoned his Ukrainian counterpart, Leonid Kozhara, urging an end to violence.
The clashes in Kiev erupted only hours after Mr. Steinmeier, who has repeatedly and bluntly urged the Ukrainian authorities to fulfill their promises to the opposition, had received the two main opposition leaders, Vitali Klitschko and Arseniy P. Yatsenyuk, in Berlin, where they also met Ms. Merkel for more than an hour.
As those meetings were held, Russia made the surprise announcement that it would lend Ukraine the next $2 billion worth of assistance in a $15 billion package that Mr. Yanukovych signed with the Kremlin in December, but which Moscow suspended when the Kiev protests did not abate.
The unusually high-level reception accorded the opposition leaders by Berlin showed how closely Germany has been tracking Ukraine’s crisis, but also illustrated how little leverage Europe’s most powerful economy has had as the tug of war between Russia and the West has unfolded over Ukraine.
Alexander Kwasniewski, the former Polish president who has been European Parliament’s special envoy to Ukraine and visited dozens of times in the past year, called the events in Kiev a “Ukrainian Tiananmen,” alluding to the 1989 Chinese military crackdown on demonstrators in Tiananmen Square in which hundreds were killed.
In comments to Poland’s RMF radio, reported by the German news agency DPA, Mr. Kwasniewski also acknowledged that Europe was relatively powerless to influence events.
“One can’t do much to stop the bloodshed,” he said.
“Diplomatic acts are like a howl in the desert.”
“I fear the thin red line has been crossed,” the former Polish president added, saying that the worst of all situations had now arisen, “where the government does not want to negotiate and the opposition does not have the strength to control all the action.”
Source: The New York Times