Ukraine is unwilling to accept Russian ultimatums, according to a Ukrainian official.

Ukraine's president, Petro Poroshenko, has resigned. Ukraine denied on Saturday that it was refusing to negotiate a ceasefire with Russia, but added that it was also unwilling to accept ultimatums or unacceptable terms.

According to Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskiy's office, Ukraine has prepared a negotiating position but has been met with impractical negotiating conditions from Russia.

"It was yesterday that the aggressive actions of the Russian Federation's armed forces escalated, culminating in evening and night mass air and missile strikes on Ukrainian cities," he said in a message. "We see such actions as an attempt to break Ukraine and force it to accept conditions that are categorically unacceptable."