KYIV, Nov 24 - Russia staged military drills in the Black Sea, south of Ukraine, on Wednesday and said it needed to sharpen the combat-readiness of its conventional and nuclear forces because of heightened NATO activity near its borders, Reuters reported.

Ukraine, which says it believes Russia may be preparing an invasion, held exercises of its own near the frontier with Belarus.

Russian investigative group posted photos and videos it said showed movements of tanks and other military vehicles in southern Russia in the past few days.

The increase of military activity on both sides follows weeks of rising tension that have raised the risk of war between the two ex-Soviet neighbors, even though Russia denies aggressive intent and Western intelligence sources have told Reuters they do not see any invasion as imminent. read more

British Foreign Secretary Liz Truss told Reuters it would be "a grave mistake from Russia" to attack Ukraine.

Estonian Foreign Minister Kaja Kallas said the European Union must make it clear to Russia that there would be a high price to pay if it acted against Ukraine, urging the EU to quickly agree on how to deter Moscow.

In Wednesday's exercises in the Black Sea, Russian fighter planes and ships practiced repelling air attacks on naval bases and responding with air strikes, Interfax reported.

Separately, the news agency quoted Defense Minister Sergei Shoigu as saying the need for Russia to further develop its armed forces was dictated by "the complicated military and political conditions in the world and the growing activity of NATO countries near Russia's borders".

He said raising the armed forces' capabilities, supporting the combat preparedness of nuclear forces and strengthening the potential of non-nuclear deterrence are among the priorities.

Shoigu complained on Tuesday that U.S. bombers had rehearsed a nuclear strike on Russia from two different directions earlier this month, and that the planes had come too close to the Russian border. The Pentagon said these drills adhered to international protocols.

Ukraine on Wednesday held what it called a "special operation" at the border with Belarus, including drone exercises and military drills for anti-tank and airborne units.

It has deployed 8,500 extra troops to its boundary with Belarus, amid fears of a potential migrant crisis and worries that the border could be used by Moscow to stage a military assault.

The Conflict Intelligence Team, an independent Russian investigative group, posted video and photos on its website showing movements of tanks, infantry combat vehicles and howitzers this month near Voronezh, about 190 km (120 miles) from the Ukrainian border, and a new concentration of army vehicles at a training ground to the south of the city.

It also posted what it said was video evidence of military trains carrying infantry fighting vehicles and armored personnel carriers in the past two days to Valuyki, 23 km from the Ukrainian border.

It said its assessment was that Russian troops would be ready for potential operations inside Ukraine no earlier than the start of next year. (rt/ez)