A Pentagon spokesman told late on Friday that the incidents had happened mainly near the border with Russia, but gave no further details.
Earlier, pro-Russian separatists seized a bus carrying international military observers, Ukrainian officials said.
Talks were under way to secure their release near the town of Sloviansk.
Russia has tens of thousands of troops deployed along its side of the border with Ukraine as pro-Moscow separatists continue to occupy official buildings in a dozen eastern towns, defying the government in Kiev.
The US and the EU appear to have moved closer to imposing further sanctions on Russia after a day of conference calls between Western leaders.
Reports say the sanctions would target individuals and entities. Under current US and EU measures, assets freezes and travel bans have targeted a number of Russian officials.
Source : BBC