Nato Secretary General Jens Stoltenberg condemned an incident where two Russian warplanes allegedly flew simulated attack passes near a US guided missile destroyer in the Baltic Sea on 12 April.
"The kind of behaviour is unsafe and unprofessional," Stoltenberg said, speaking outside the residence of the British Prime Minister, 10 Downing Street, in London on 14 April.
"That's the reason why I have, for a long time now, underlined the importance of more transparency, more predictability and increased efforts on risk reduction because we have seen substantial, significant risk increases in military activity along our borders and we have to avoid incidents and accidents. And if they happen we have to make sure they don't spiral out of control," he added.
The Nato Secretary General was received by David Cameron at Downing Street during the morning on 14 April.