Environmental campaigners in America are concerned about a new danger to the world's whale population.

It's sonar - powerful sound waves used by submarines and ships to track activity underwater.

Research has shown that the sonar waves can be heard by whales and dolphins - and campaigners believe it may be causing the whales to beach.

The American navy starts using a controversial new low-frequency system next month, even though some types of sonar have already been blamed for causing strandings. Breakfast's Jane O'Brien reports from the coast of Cape Cod, in the USA

click here to watch Jane O'Brien's report in full

It was a stranding of rare beak nosed whales in the Caribbean which gave scientists their first proof of the harm sonar can inflict.

In spite of the efforts of local people, six whales died.

post-mortems showed bleeding

Post mortem examinations found that their inner ears had been severely damaged

Each whale ear was sliced into a thousand sections and stained to reveal bone and blood. Red areas show the bleeding caused by exposure to military sonar.

At the moment, we think sonar precipitated the strandings, " says Dr Darlene Ketten.

Ketten: believes sonar caused the stranding

"The animals were driven to strand by the stress of being exposed to this particular sonar."

The sonar implicated in this stranding was mid-frequency.

The new system is lower and louder and travels much further. The noise is the equivalent of a jumbo jet taking off.

The American Navy says it needs a more powerful device to keep track of potentially threatening submarines. But it's continuing to research into the effects of sonar on marine life.

The US navy says there's no conclusive proof

Norman Polmar, who's a consultant to the United States Navy says:

"There is no finite correlation between low frequency sonar and negative effects on marine wildlife.

"Can marine life hear it? Yes. Does it have a major effect? We don't know."

This uncertainty alarms wildlife groups. They're now threatening legal action to protect sea-life from the sonar.

Whale campaigner Jo Reynolds' view is unequivocal: "Very intense sound can have severe consequences, even death," he says

campaigners are considering legal action

"It causes us great concern that the navy proposes to deploy one the loudest sound systems devised by man over 80 per cent of the world's oceans without really understanding what the implications are."

Whales are already under threat from pollution, fishing nets and collisions with boats. Although the research is inconclusive, conservationists say we should avoid any further risks to them.