U.S. Vice President Joe Biden said the United States will work with its partners to impose "further costs on Russia" if it does not use its influence to help halt separatist violence in eastern Ukraine.
The White House said Biden spoke by telephone Wednesday with Ukrainian President Petro Poroshenko, and that the two promised to "stay in touch" ahead of a meeting of European Union leaders next week.
The U.S. and EU imposed sanctions against a group of Russian individuals and companies after Russia annexed Ukraine's Crimean Peninsula in March.
Pro-Russian separatists have for months mounted a rebellion in eastern Ukraine, which the United Nations said Wednesday has involved killings, torture and abductions to instill a "climate of fear."
Gianni Magazzeni, a U.N. human rights office official, said the situation has created challenges for those caught up in the fighting.
"This has to so with access to public services whether it's schools, medical services, whether it's food, whether it's electricity, water. But it's also a question of a total breakdown in law and order, the inability to get protection when it comes to ill treatment, detention, abductions and possible also torture and executions that are, and we are reporting, are taking place in this pocket areas in the East in the course of the past month," said Magazzeni.
U.N. human rights chief Navi Pillay urged armed separatists to stop actions she says are leading to misery and destruction in the Donetsk and Luhansk regions. She said "the time has come to put down the guns and talk."
President Poroshenko has proposed instituting a unilateral cease-fire as part of a peace plan to address the conflict.
Russian news agencies quoted Poroshenko on Wednesday as saying the cease-fire would give separatist fighters the opportunity to disarm and those who want to leave the country a chance to get out.