Russia is holding naval war exercises and surface-to-air missile drills in the Black Sea region amid tensions with Ukraine and a U.S.-led alliance over the capture of three Ukrainian naval vessels and their crew off the coast of Moscow-annexed Crimea last month, the Kremlin’s defense ministry reportedly announced on Tuesday.

Also on Tuesday, NATO Secretary General Jens Stoltenberg reportedly declared that the U.S.-led military alliance had intensified its presence in the Black Sea region in response to what he described as “Russia’s aggressive actions.”

“We will continue to assess our presence in the region,” he proclaimed.

Citing a statement from the ministry, Reuters reported on Wednesday:

Two submarines, the Rostov-on-Don B-237 and the Stary Oskol B-262, practiced emergency deployments for detecting, accompanying and destroying sea and coastal targets with rocket fire…Submarine crews from the Black Sea fleet were also due to practice deep dives and to work on techniques such as emergency surfacing, it said … Separately, Pantsir medium-range surface-to-air missile systems in the east of Crimea practiced detecting, identifying and shooting down aerial targets, Interfax news agency cited a spokesman for the Black Sea Fleet as saying.

This week, Fox News, citing satellite images, reported that the Russian military deployed additional S-400 surface-to-air missile batteries to Crimea.

In 2014, Russia annexed the Crimean peninsula, triggering Western sanctions.

The United States and NATO continue to lend support to ally Ukraine in a war against Russian-backed militants in the east of the country that has killed an estimated 10,000.

Reuters acknowledged that the Russian defense ministry did not provide a start or end date for the war games. Moscow also failed to indicate if the submarine exercises were part of larger-scale military exercises in the Black Sea and on the Crimean peninsula.

Reuters learned from a navigation warning to shipping published by Turkey’s navy that “firing exercises” are expected in the region from December 3 to December 9.

The drills come amid heightened stress between Russia and the West over Russia’s seizure of two small Ukrainian military ships, a tugboat, and their crews on November 25.

In late November, the NATO-backed Ukrainian military held artillery drills near Azov Sea that borders the Crimean peninsula.

“Ukrainian armed forces carried out tactical drills on Thursday [November 29] near the Azov Sea coast as tensions between Ukraine and Russia soar,” Global News reported. “The drills, focused on air defense of the city of Mariupol, coincided with Russia’s deployment of S-400 missile defense systems along Crimea’s bottleneck Ukraine border.”

On Tuesday, BBC, citing an unnamed U.S. commander, suggested that Russian military drills near Ukraine are not a prelude to conflict, but the war itself.

BBC reported:

Warfare is changing. Indeed, the once-clear boundary between war and peace is dissolving. We hear a lot about military demonstrations, exercises, cyber attacks and information operations. Traditionally, these have been seen as a prelude to conflict. But what if they are actually the war itself, as one senior US commander recently asked? In other words, you no longer have to risk full-scale warfare; you just use a variety of tools to apply pressure to achieve the same ends. In this light, Russia is already having some success against Ukraine.

Under U.S. President Donald Trump, the American military is moving away from its war on terror to combat foes like Russia and China.