HAVANA (Reuters) - It all starts with a description given over a mobile phone: "Look for a woman with long blonde hair, blue jeans, silver heels and a black T-shirt arriving on the next flight from Miami."

When the woman emerges from Havana's international airport pushing a cart loaded with bulky black duffel bags, she is greeted effusively by a man she has never seen before.

"They hug as if they had known each other all their lives. Once in the parking lot, the woman hands over the bags and says goodbye," says Yanet, a Miami resident.

She is describing the tactics of growing numbers of human "mules" who regularly travel between the United States and Cuba carrying in their bags loads of clothes, food, consumer goods, electrical appliances and millions of US dollars to the communist-ruled Caribbean island. They deliver the goods for a fee or free ticket, often to complete strangers.

"The system works beautifully," said Yanet, making her second trip as a "mule" to Havana in less than a month.

"But you have to stage a little show because you never know who may be watching," she added.

This burgeoning informal commerce between two neighbours whose governments have maintained a Cold War-era enmity for half a century belies the 48-year-old US trade embargo against Cuba — but also reflects recent relaxations of it.

Since 1962, the US embargo's intended aim has been to force the Cuban government to abandon its communist rule.

But informal trafficking of cash and goods to Cuba has boomed since US President Barack Obama last year lifted restrictions on Cuban Americans traveling to their homeland and significantly increased the amount of money they could take.

His calibrated measures, part of a process of promoting "people-to-people" contacts Washington believes can foster political change in Cuba, also increased the type of consumer items that could be included in gift parcels for Cuba.

Also authorised under a telecoms initiative was the export or re-export to Cuba by visitors of donated personal telecoms devices, such as mobile phones, computers and software.

Travellers to Havana were already able to bring parcels of food and medicines, and the embargo has for some years allowed the export of US farm products to the island.

On the US side, from where daily two-way charter flights ferry more and more Cuban Americans to Cuba on family visits, there is significant tolerance for passengers to load up with consumer goods.

But the mules also need to outsmart tight Cuban customs restrictions, where taxes are levied for baggage over certain limits and luggage contents are frequently inspected.

Chronic scarcity and the high prices of the narrow range of imported goods that are sold in Cuba's state-run dollar stores have prompted thousands of Cubans to use the human "mules" to import everything from clothing to toiletries, electronics and money.