THESSALONIKI, Greece -- In the rugged, herb-scented mountains of northwestern Greece, where the border with Albania is a snaking invisible line, trouble is brewing over tea -- the wild herbal variety.

Greek authorities and conservationists say bands of impoverished Albanians are making regular cross-border forays, illegally harvesting donkey-loads of herbs and medicinal plants. They mostly pick mountain tea -- also called ironwort -- hawthorn and even primrose, but they are also destroying rare and endangered species in the process.

The looters then sell the herbs for export to pharmaceutical or cosmetics companies, a business that nets Albanian wholesalers tens of millions annually.

It's illegal in Greece to pick more than a tiny quantity of wild herbs for personal use in traditional infusions. That ban doesn't exist in Albania, one of Europe's poorest nations. But, more significantly, the plants are usually uprooted in the looters' haste to pick as much as possible and be off undetected. This stops natural regeneration, threatens delicate ecosystems and leaves entire mountainsides denuded.

Albanians contend the herbs are there and the Greeks don't pick them, so why shouldn't somebody profit?

Christos Toskos, an environmentalist in Greece's Kastoria border area, says the depredations have increased over the past five years, with incursions now coming on a daily basis.

"There is very large destruction in areas covering thousands of acres," he said.

Vassilis Filiadis, who grows his own herbs in Kastoria, lamented the fate of an old wild ironwort patch in the Grammos mountains.

"It covers about 3 square kilometres (740 acres). In past years, the mountain tea grew there like a sea. The plants formed waves," he told The Associated Press. "I went this year and was shocked, it's all been uprooted."

Greece's flora is among the richest in Europe, with about 6,500 native plant species.

In targeted operations over the last few months, Greek police have arrested at least ten Albanians and seized dozens of kilograms of herbs. In one case in late June, three people were caught with 136 kilograms (300 pounds) of ironwort loaded on two horses and a donkey.

Albanian exporters pay illegal gatherers up to 6 euros ($6.80) a kilogram (2.2 pounds) for ironwort and 7 euros a kilogram for hawthorn, Greek officials say.

"They illegally enter Greece and quickly gather the plants to avoid being seen," said Brigadier-General Panagiotis Ntziovaras, head of police for the border region of western Macedonia.

Those caught have been given suspended prison sentences of one or two months and been deported.

Many poor Albanians are crossing the mountains into Greece this year because of an herb shortage in Albania due to freezing temperatures last winter, said Filip Gjoka, president of Albania's Association of Medicinal & Aromatic Plants and owner of an herb and spice trading company.

He said they sometimes take whole families and camp in the mountains with their horses or mules.

"There are a lot of herbs in Greece, where they are not collected due to labour force shortages or lack of interest," Gjoka told the AP. "We here collect those herbs, and these people take the risks to support their families. They can bear a few months of jail since there are no other jobs."

In 2016, 24 Albanian companies exported some 17,000 tons of medicinal and aromatic plants and herbs -- 186 varieties -- worth a total of $40 million. They process only about 30 per cent of that amount in five factories and export the rest raw.

The U.S. is a main importer, while others include France, Germany, Spain and even Australia.

Kastoria agriculturalist Dimitris Natos said the international market for herbs, particularly for use in cosmetics and foods, is expanding rapidly.

"Annual turnover growth is in the double digits, at around 15 per cent," he said.

Gjoka said the Albanian companies employ 10,000 workers and another 80,000 people as independent contractors for whom seasonal herb picking is their only source of income.

Eleni Maloupa, director of Greece's Institute of Breeding and Plant Genetic Resources in Thessaloniki, says some of the 14 kinds of ironwort that grow in Greece are threatened with extinction and there is a blanket ban on their collection, even in small quantities.

She said Greek and Albanian authorities should co-operate to solve the problem, as Greece has already done with neighbouring Macedonia.

"The increased arrests may perhaps discourage (illegal harvesters) but I believe we should use all available means, such as drones or cameras, to control the border and illegal plant picking," she said.

Llazar Semini in Tirana, Albania, and Nicholas Paphitis in Athens, Greece, contributed to this story.