Russia on Wednesday accused Ukraine of preparing a series of terrorist attacks and of igniting border clashes on the disputed Crimean Peninsula, the latest in a worrying trend of violent incidents along the line of contact between the two countries.

In a sign that the accusations could lead to an escalation, Russian President Vladimir Putin, after a meeting in Moscow with his Armenian counterpart, declared that Ukraine had “gone over to the practice of terror” and, referring to the death of two Russian servicemen in the alleged attacks, said Russia “will not walk past something like this.”

“This is a very dangerous game,” he said, saying security measures on the peninsula would be increased following the attacks.

Ukraine denied the accusations and alleged that Russia was building up its forces in Crimea to turn the peninsula into an “isolated military base.”

The accusations came after several days of reports of border closures and heightened military presence from Russian-controlled towns south of the administrative boundary between Crimea and Ukraine. They also come amid a rise in shooting and shelling attacks between Ukraine and pro-Russian separatists in the country’s southeast, as well as mounting casualties.

In its statement Wednesday, Russia’s Federal Security Service said that it had disrupted a secret network of pro-Ukrainian saboteurs who planned to attack crucial infrastructure on the peninsula with explosives and other weapons. An employee of the agency died as the suspects were being arrested, it said.

The agency also claimed to have prevented two more attempts by Ukrainian saboteurs to cross the administrative border under the protection of Ukrainian artillery, which killed a Russian serviceman.

Russia annexed the peninsula in March of 2014 following a revolution in Kiev, Ukraine’s capital.

In a statement, the Ukrainian Defense Ministry called the Russian accusations “an attempt to justify the dislocation and aggressive actions of military units of the Russian Federation on the territory of the currently occupied peninsula.”

The Organization for Security and Cooperation in Europe, which maintains a monitoring mission in Ukraine, reported unusual closures of border crossings and, on the eve of the supposed clashes, “noted that border guard personnel were on heightened alert, carrying assault rifles and continuously searching the area with binoculars.”

This week, online activists have published videos on social networks of Russian trains carrying army vans to the Kerch Strait, the waterway between Crimea and southern Russia. There have also been isolated reports of Internet outages in the Crimean border city of Armyansk, where the fighting is supposed to have taken place.