Ukrainian military officials are warning against a possible Russian military invasion at "any minute." "The occupiers are conducting manoeuvres and we should understand that at any minute, at any hour, they could start a large-scale or small-scale attack," said Andriy Lysenko, military spokesman for Ukraine’s president Petro Poroshenko.

Ukraine is deploying more military equipment and forces to bolster its army units located near the border with Russian-annexed Crimea in response to a Russian military building up in the disputed peninsula, Ukraine Today reported on Tuesday.

The news site cited confirmation by Vladyslav Seleznev, the speaker of Ukraine's general staff of the armed forces, of the move. Local residents near the border said they spotted Ukrainian heavy military equipment, such as anti-tank rocket launchers, heading their way, according to the website.

Seleznyov said Ukraine was responding to Russian military activity in Crimea.

"The Kherson border guards spotted nine Russian Mi-8 helicopters and two drones moving above the temporarily occupied territory of Crimea, along the administrative border with mainland Ukraine," Ukraine's State Border Service stated. "The relevant units of armed forces were notified."

Tensions have run high in the area, sparking fears that violence between the two countries could re-erupt. The Crimean Human Rights Group posted photos on its website showing Russian military trucks being transported by train on August 6 near Kerch, a port in Eastern Crimea near the border with Russia proper. Ukraine has seen a surge in violence in the eastern regions of the country held by pro-Russian separatists, with three Ukrainian soldiers killed and four wounded in the last 48 hours, reported Newsweek on Tuesday.

Russian authorities closed the border between Crimea and Ukraine proper for several hours on Sunday, Radio Free Europe reported. Russia annexed Crimea in March, shortly after occupying the peninsula and forcing out Ukrainian troops.