A Ukrainian soldier fires a machine gun during 2016 clashes with pro-Russian separatists in Avdiivka
Kiev (AFP) - Ukraine on Friday accused Russia of plotting further unrest as the international community sought to ease tensions after the two sides ramped up security around the disputed Crimean peninsula.
Moscow also said it has delivered its most advanced S-400 air defence system to Crimea, which it annexed in 2014. The system will become fully operational in December, the military said.
EU foreign affairs chief Federica Mogherini reiterated the bloc's support for Ukraine, saying it did not recognise the "illegal annexation of Crimea", while urging both sides to avoid any actions which could further escalate tensions.
Russia's FSB security service has said it thwarted "terrorist attacks" in Crimea last weekend by Ukrainian military intelligence and beat back armed assaults, claims Kiev has fiercely denied.
Both sides have in response boosted security around the region as the simmering feud over Crimea has unexpectedly flared up again, prompting fears of a wider conflict.
Ukrainian troops near Crimea and along the frontline with pro-Russian separatists in two other eastern regions have been placed on high alert, and Kiev's military intelligence said "the enemy is planning large-scale provocative actions".
It also accused Russia of increasing its forces, replenishing munitions and building up military hardware in the war-scarred east of Ukraine.
Prime Minister Dmitry Medvedev followed up angry accusations from President Vladimir Putin by branding Kiev's alleged raids in Crimea "a crime against the Russian state and the people of Russia".
He warned that Putin could sever diplomatic ties with Ukraine if "there remains no other way to influence the situation", Russian news wires reported.
Ukraine's security service has raised its terrorist threat level to the highest around Crimea and the eastern regions of Donetsk and Lugansk, where pro-Russian rebels have been fighting government troops for more than two years.
- TV 'confessions' -
Kiev's alleged attempts at armed incursions into Crimea saw a Russian security service officer killed in clashes while arresting "terrorists" at the weekend, Moscow said.
And a Russian soldier died in a firefight with "sabotage-terrorist" groups sent by the Ukrainian military on Monday.
Ukraine's national security council chief Oleksandr Turchynov on Friday dismissed the claims, saying Moscow was trying to cover up deadly shootouts between Russian forces "who traditionally abuse alcohol".
In a move likely to fuel the dispute, Russian state TV aired footage of the interrogations of several alleged infiltrators, who authorities say have confessed to working for Ukraine's military intelligence.
Kiev had previously said one of the men being held in Russia had been abducted and was a "hostage".
The international community has tried to tamp down the rising tensions over fears they could kill off a stalled peace process for eastern Ukraine signed in the Belarussian capital Minsk in February 2015.
At a meeting of the UN Security Council on Thursday, Ukraine asked Russia to provide evidence to back its accusations, which President Petro Poroshenko had called "senseless and cynical".
Putin has said a mooted meeting with Poroshenko and mediators German Chancellor Angela Merkel and French President Francois Hollande at next month's G20 summit in China would now be "senseless".
A German government official told AFP on Thursday that Putin's rejection of such a meeting was a "sign that Russia is less and less counting on a political solution to the crisis".
The French foreign ministry meanwhile said minister Jean-Marc Ayrault told Russian counterpart Sergei Lavrov that France and Germany would continue efforts to implement the Minsk agreements, which include the four countries.
The United States also called on both sides to show restraint, with State Department spokeswoman Elizabeth Trudeau saying Thursday that Washington was "extremely concerned".