Prime Minister Dmitry Medvedev says Russia could break diplomatic ties with Ukraine over reported security incidents in Crimea, something it did not do even after annexing Crimea or throwing its support behind separatist rebels in the east.

State news agencies on Friday quoted Medvedev as saying that he wouldn't like the ties to be severed but "if there is no other way to change the situation, the president could take this step."

Despite the 2014 Russian annexation of Crimea and fighting in eastern Ukraine, Kiev and Moscow did not break diplomatic ties.

The announcement comes after Ukraine put its troops on combat alert Thursday along the country's de-facto borders with Crimea and separatist rebels in the east, amid an escalating war of words with Russia over Crimea.