Tensions peak between Russia and Ukraine over Crimea as Moscow accuses Kiev of attempting incursions in the annexed peninsula.
Tensions around Crimea are high after the Russian Navy announced plans to hold exercises in the Black Sea to practice repelling attacks.
The exercise comes after Moscow claimed that Ukraine was attempting an ‘incursion' into the annexed peninsula.
Russia's defence ministry said on Thursday that the scenario would be based on pushing back an underwater attack by "saboteurs" from the sea.
"The people who seized power in Kiev ... have switched to terror tactics instead of searching for ways for a peaceful settlement," Russian President Vladimir Putin told a news conference, saying Russia would not let such actions pass without a response.
In an immidiate rebuttle, Ukrainian President Petro Poroshenko called the allegations as "excuses" to escalate towards a war in the Crimean peninsula, which Moscow annexed from Ukraine in 2014.
"Russian accusations against Ukraine of terrorism in occupied Crimea sound as preposterous and cynical as the statements of the Russian leadership about the absence of Russian troops in the Donbass," Poroshenko responded, adding that Russia's claims were "preposterous".
Poroshenko stated that troops in Ukraine's rebel-held eastern Donbass region would be brought to the highest level of combat readiness.
Later on Thursday he expressed his wish to speak directly with Putin and several Western leaders amid a spike in tensions between Kiev and Moscow.
Reiterating Poroshenko's words, Ukrainian military spokesman Oleksandr Motuzyank said that Russian accusations that Ukraine attempted incursions in Crimea did not correspond with reality.
Motuzyank stated that Ukraine did not commit any armed provocations and further denied claims that defence ministry intelligence staff were detained in the "occupied" Crimea.
"Those actions and statements by the Kremlin were made in order to discredit Ukraine, to stall the Minsk agreements and to escalate conflict in eastern Ukraine which was caused by the Russian aggression," Motuzyanyk added.
According to Ukrainian border guards, Russian troops have amassed more troops in recent days and equipped themselves with more modern gear on Ukraine's border with Crimea.
The United States' ambassador to Ukraine, Geoffrey Pyatt also commented on the tensions, saying that the US has seen nothing so far that corroborates Russian allegations of a Ukrainian incursion into Crimea.
1/5 US government has seen nothing so far that corroborates Russian allegations of a Crimea incursion & Ukraine has strongly refuted them.— Geoffrey Pyatt (@USAmbGreece) August 11, 2016
"Russia has a record of frequently levying false accusations at Ukraine to deflect attention from its own illegal actions," Pyatt wrote on Twitter.
2/5 Russia has a record of frequently levying false accusations at Ukraine to deflect attention from its own illegal actions.— Geoffrey Pyatt (@USAmbGreece) August 11, 2016