French police have closed beaches in southwest France as packages of cocaine and other drugs continue to wash up daily along the Atlantic coast, with nearly 900 kilograms discovered since mid-October.

Officials said the cocaine was extremely pure at some 83 per cent, warning people who find the packages not to touch them. That has not stopped some from trying to get their hands on the drugs, with a street value in the millions of euros, prompting police to close beaches and start carrying out patrols.

On Monday (local time), a 17-year-old was caught with 5kg of cocaine at Lacanau, a closed-off surfing beach near the southwestern city of Bordeaux — he had come from Toulouse, a three-hour drive away.

A half-dozen other beaches in the area have been closed as well, with police stopping walkers for searches and also checking cars leaving nearby parking lots. A police helicopter was also being used along the 125-kilometre stretch between Cap Ferret and Soulac-sur-Mer, which includes Lacanau.

A woman who gave her name as Martine failed to notice the closure signs at the Porge beach on Monday (local time), when officers told her and a friend to leave. "When we turned around we saw a package next to the water," she said, saying they alerted the police, who estimated it weighed some 4kg before sealing it in a plastic bag.

"It would have been tempting to take it, but we're honest," she said.

Philippe Astruc, the prosecutor in the western city of Rennes, where officials are coordinating the searches, said in a statement "we've recovered 872 kilos".

It was a sharp jump from just two days ago, when 763kg were reported. "Intense surveillance has been set up by different services, notably the customs agency, all along the French coast," Mr Astruc's office said.

Officials have reported the drugs turning up on beaches near Nantes all the way to the resort town of Biarritz, more than 500km to the south.

French investigators are working with European counterparts as well as the US Drug Enforcement Agency to try to determine why the drugs are washing up.

"There are several hypotheses, but for the moment we think it's likely they were jettisoned because of a mechanical problem or during a storm," Mr Astruc said.

Sud-Ouest reported over the weekend that some packages found on the beach at Arcachon, southwest of Bordeaux, were marked 'Diamante' or 'Brillante'.

Packages of cocaine marked with the same words also reportedly washed up in Florida during hurricane Dorian in September, it said.

Police seized a record of more than 140 tons of cocaine across Europe in 2017, the most recent data available from the EU's European Monitoring Centre for Drugs and Drug Addiction, published in June. Most was seized in Belgium and Spain, along the main port of entry for the drug, and its purity has increased markedly over the past 10 years, the agency said.

// AFP