President Poroshenko said a prisoner exchange could begin as early as Saturday.
Leaders of the NATO countries would respond to Russian actions in Ukraine by providing security assistance.
Some four months of fighting have left over 2,000 people dead and forced some 285,000 to flee their homes.
Tymoshenko was jailed in 2011 for abuse of office over a gas deal with Russia but her supporters and Western leaders regarded her as a political prisoner.
Kiev parliament sets early election for May 25, Opposition leader Tymoshenko freed from prison.
The agency said in a note that the downgrade reflects our view that political situation in Ukraine has deteriorated substantially.
At least 17 people were killed in fierce clashes in Kiev between protesters and riot police.
At least 16 people were killed in fierce clashes in Kiev between protesters and riot police.
Ukraine’s political crisis worsened when 25 people were killed and 241 injured in a fiery battle between anti-government and police.
In the audio, Nuland and Pyatt discuss their views of various opposition figures and whether or not they should take positions in the government.
Meanwhile,US voiced ‘disgust’ at the Ukraine authorities for crackdown on protestors.