The Russian military has conducted massive drills in Crimea involving dozens of navy ships, hundreds of warplanes and thousands of troops in a show of force amid tensions with Ukraine.

The manoeuvres were described as the largest since Russia annexed the Black Sea peninsula from Ukraine in 2014 and threw its weight behind separatist insurgents in eastern Ukraine.

The exercise comes amid increasing violations of a ceasefire in Ukraine's east and a massive Russian troops build-up on the border with Ukraine that raised Western concerns.

The Russian Defence Ministry said the Crimean manoeuvres involve more than 60 ships, over 10,000 troops, around 200 aircraft and about 1,200 military vehicles.

The exercise featured the landing of more than 2,000 paratroopers and 60 military vehicles on Thursday.

Fighter jets covered the airborne operation.

Russian defence minister Sergei Shoigu flew in a helicopter over the Opuk firing range in Crimea to oversee the exercise.

Last week, Russia announced that it would close broad areas of the Black Sea near Crimea to foreign navy ships and state vessels until November, a move that drew Ukrainian protests and raised Western concerns.

Russia also announced restrictions on flights near Crimea this week, arguing that they fully conform with international law.

Ukrainian foreign minister Dmytro Kuleba warned on Tuesday that the Russian build-up across the border is continuing and is "expected to reach a combined force of over 120,000 troops" in about a week and urged the West to beef up sanctions against Moscow.

Moscow has rejected Ukrainian and Western concerns about the build-up, arguing that it is free to deploy its forces anywhere on the Russian territory and claiming that they do not threaten anyone.

But at the same time, the Kremlin sternly warned Ukrainian authorities against trying to use force to retake control of the rebel east, where seven years of fighting have killed more than 14,000 people, saying that Russia could be forced to intervene to protect civilians in the region.

Amid the tensions, Ukrainian President Volodymyr Zelenskyy on Wednesday signed a law allowing the call-up of reservists for military service without announcing a mobilisation.

The new law will allow for quickly equipping the military with reservists, "significantly increasing their combat effectiveness during military aggression", Mr Zelenskyy's office said in a statement.