Kyiv accused Russia of seeking to provoke an escalation of the conflict in eastern Ukraine, saying Moscow has bolstered separatist forces in the region with fresh deliveries of ammunition and military hardware.

The assertion came as tensions between Moscow and Kyiv continued to rise after Russia claimed that Ukraine had tried to send “saboteurs” into Crimea to carry out “terrorist” attacks against infrastructure on the Russian-annexed peninsula -- an allegation Kyiv says is “preposterous.”

Russia's Defense Ministry on August 12 announced the deployment of S-400 air-defense missile systems -- which Moscow has touted as state-of-the art weapons -- in Crimea. The military had pledged last month to deploy the system on the peninsula. And Prime Minister Dmitry Medvedev raised the prospect of severing diplomatic relations with Kyiv in order to “sober up” Ukraine.

“I would not want that to happen, but if there is no other option left to impact the situation, the president [Vladimir Putin] could make such a decision,” Medvedev said in response to a question. He noted that diplomatic ties between Russia and Georgia were cut off when they fought a brief war in 2008.

Russia’s accusation of a Ukrainian plot to destabilize Crimea, which Moscow seized from Ukraine in March 2014, added to tension following weeks of increased fighting between government forces and the Russia-backed separatists who hold parts of the Donbas region in eastern Ukraine.

Each side is blaming the other for the increased tension.

Ukraine's military intelligence service, which has categorically denied Russia’s claims, alleged on August 12 that Russia was planning "large-scale provocative actions through the contact line in Ukraine’s east” -- a reference to the line separating government and separatist forces.

Russia “will then accuse Ukraine of not complying" with the Minsk agreement, a Western-brokered peace deal for eastern Ukraine.

The accusation came a day after President Petro Poroshenko put Ukraine’s forces on the highest alert level in both eastern Ukraine and along the administrative boundary between mainland Ukraine and Crimea.

The Foreign Ministry in Kyiv on August 12 demanded that Russia give monitors from the Organization for Security and Cooperation in Europe (OSCE) access to Crimea as well as greater access to separatist-held areas in eastern Ukraine, saying that Moscow is obliged to do so under existing agreements.

It also called for monitors from the International Red Cross and the United Nations' human rights monitoring mission to be given access to Ukrainian detainees who are in the custody of Russian authorities.

Also on August 12, Russian lawmaker Olga Kovitidi said the Russia-installed government in Crimea has cut off Internet access in the northern part of Crimea, which is closer to the mainland, "for security reasons."

Evidence pointing to a buildup of Russian military equipment in northern Crimea in the past week emerged on social media both before and after Russia made its claim about a Ukrainian plot in Crimea on August 10.

A report by the Digital Forensic Research Lab of the Atlantic Council, a U.S.-based think tank, said that a “deluge of photographs and videos" had appeared online since August 7 that indicate a “mass mobilization” of Russian military equipment throughout Crimea. It said the deployments include the movement of truck-mounted Bastion-P coastal missile-defense systems.

At the same time, the report cast doubts on claims by Russia’s Federal Security Service (FSB) that artillery positioned in Ukraine's Kherson region had shelled Russian positions in northern Crimea on August 8, killing one Russian soldier.

It said that there were few reliable local witness accounts of a Ukrainian artillery attack on Russian forces in Crimea on VKontakte, Odnoklassniki, Twitter, or other social media -- and that no photographs or video of gunfire and shelling had emerged.

Russia said on August 11 that it was stepping up security in Crimea and also announced a three-day naval exercise to practice repelling underwater attacks on the Black Sea Fleet, which is based in the Crimean port of Sevastopol.

The FSB says it thwarted an incursion by Ukrainian “saboteurs” between August 6 and August 8 and that it detained several suspects, including both Ukrainian and Russian citizens.

The U.S. ambassador to Ukraine, Geoffrey Pyatt, said on Twitter on August 11 that the U.S. government "has seen nothing so far that corroborates” the Russian allegations. European Union officials have also cast doubt on the Russian claims.

Ukrainian military spokesman Oleksandr Motuzyanyk says Russia’s actions and statements are aimed at stalling the peace process under the Minsk agreements, discrediting Ukraine, and escalating conflict in eastern Ukraine.

Ukraine's ambassador to the United Nations, Volodymyr Velchenko, charged on August 11 that some 40,000 Russian troops are now amassed in Crimea and along Russia’s border with eastern Ukraine.

With reporting by AFP, Reuters, AP, TASS, and Interfax