The United States has seen nothing so far that corroborates Russian allegations of a Ukrainian incursion into Crimea, the US ambassador to Ukraine, Geoffrey Pyatt, says.
Pyatt also said US sanctions in relation to Russia's annexation of Crimea would remain in place until the territory was returned to Ukraine.
Russian President Vladimir Putin accused Kiev on Wednesday of using terrorist tactics to try to provoke a new conflict and destabilise Crimea, a region Moscow annexed from Ukraine in 2014.
Russia said it had thwarted armed Ukrainian attempts to get saboteurs into the peninsula.
"Russia has a record of frequently levying false accusations at Ukraine to deflect attention from its own illegal actions," Pyatt wrote on Twitter.
"US government has seen nothing so far that corroborates Russian allegations of a "Crimea incursion" & Ukraine has strongly refuted them."
Meanwhile, Russian President Vladimir Putin has held a meeting with his Security Council to discuss additional security measures for Crimea after the clashes on the contested peninsula, the Kremlin said.
Russia was also considering tightening security at the land and sea borders with Ukraine.
The Russian Navy was planning to hold exercises in the Black Sea to practise repelling underwater attacks by saboteurs, Russian news agencies on Thursday cited the defence ministry as saying.
The scenario would be based on pushing back an underwater attack by saboteurs from the sea, the ministry said.
