MOSCOW (AP) — Russia is accusing Ukrainian officials of barring crews of Russian commercial airlines from going outside of their aircraft in Ukrainian airports.

Russia's foreign ministry said Wednesday that Ukrainian border guards have been barring cabin crews of Aeroflot, the state-controlled Russian airline, from leaving the aircraft. The ministry said the decision violates international laws and ultimately "poses a threat to the safety of civil aviation" because the crews cannot rest properly.

The ministry said that it had sent protest notes twice to the Ukrainian embassy in Moscow.

Ukraine denies the allegation.

Tensions between Moscow and Kiev have been rising ever since Russian forces occupied the Crimean peninsula and stayed there throughout the referendum last week where votes overwhelmingly supported Russia's annexation.