Eleven adults and one infant calf died in a “targeted and efficient” attack highlighting the growing professionalism of poachers bankrolled by international criminals supplying soaring demand for ivory in the Far East.

Six of the animals lay in one heap, their tusks hacked out with machetes.

None of the family group managed to flee further than 300 yards before they were gunned down and their ivory removed.

The calf, less than a year old, is believed to have been crushed by its dying mother as she fell to the ground.

“It is unimaginable, a heinous, heinous crime,” said Paul Udoto, spokesman for the Kenya Wildlife Service (KWS).

“We have not seen such an incident in recent memory, it’s the worst single loss that we have on record, and our records go back almost 30 years.

“These were professional killers. The attack was targeted and efficient.”

The poachers, armed with automatic rifles, had already fled but there were hopes last night that a massive search involving foot patrols, a dozen vehicles and three aircraft could still find them.

“Every possible resource is being deployed to track down these criminals,” Mr Udoto said. “They will feel the full force of the law.”

But the area where the elephants were killed, in the north of Kenya’s largest wildlife reserve, Tsavo East National Park, is sparsely populated, has few roads, and lies close to Kenya’s border with Somalia.

Privately, conservationists said they feared the poachers and their haul of 22 tusks, worth an estimated GBP175,000 on the Asian market, would already have escaped.

The attack was the latest in a surge of elephant deaths that has seen the number of the animals killed for their ivory in Kenya increase sevenfold in five years, from fewer than 50 in 2007 to 360 in 2012, according to KWS figures.

The increase has led many wildlife experts to declare the current situation a crisis worse even than the mass slaughter of Africa’s elephants in the 1970s and 1980s, which led to a global ivory trade ban in 1989.

“Now the situation is far graver, because we have fewer elephants left, but the demand for ivory is far greater,” said Iain Douglas-Hamilton, founder of the British and Kenyan organisation Save The Elephants.

“The only thing that will radically alter the situation now is somehow to lower that demand.”

During the last six weeks, 20 elephants were found dead with their tusks hacked out in the Samburu ecosystem of northern Kenya alone. Three females were killed close to the Amboseli National Park in October.

Experts predict that many more are killed in the wilderness and their carcases never found.

Across Africa, the situation is the same, especially in Cameroon, the Central African Republic, Tanzania and the Democratic Republic of Congo.

Teams at air and seaports in East Africa and the Far East seized more illegal African ivory in 2011 than at any time in the past, as its soaring price in the Far East drove a surge in poaching.

The figures for 2012, not yet fully collated, are expected to be worse.

Two average 10lb tusks from an adult female elephant are now worth more than GBP12,000 in China, close to double their value a decade ago. The new demand is driven by the country’s booming middle class for whom carved ivory and tusk trinkets are a sign of wealth.

Occasional “one-off sales” to China and Japan of stockpiled ivory from southern Africa, most recently in 2008, are also blamed for restarting a market that had been dormant since the trade was banned.