The Incas chewed it in sacred ceremonies but the United States will be rather less respectful of the coca leaf in a modern ritual: battling Bolivia over drugs policy.

The US is expected to push this week for a continued international ban on coca leaf chewing in its latest clash with Bolivia over the Andean plant, Erythroxylum coca.

US diplomats are due to file a formal objection to Bolivia's attempt to amend a half-century-old UN ban, claiming it would promote the raw ingredient for cocaine and undermine the "war on drugs".

President Evo Morales, an Aymara Indian and former coca grower, called the prohibition, enshrined in the 1961 Single Convention on Narcotic Drugs, absurd and an affront to Andean culture.

The leaf, a mild stimulant, has for centuries been chewed and brewed in tea to combat hunger, fatigue and altitude sickness. The Aymara and other indigenous groups use it in religious and cultural ceremonies. "How can it be possible that the coca leaf, which represents our identity, which is ancestral, be penalised," Morales said last week.

Both sides have clashed repeatedly over the leaf since Morales became Bolivia's first indigenous president in 2005 on the promise of championing a culture long oppressed by European settlers and US-led "imperialism".

Morales expelled Drug Enforcement Administration agents, accusing them of spying, and asked the UN to decriminalise the leaf and bring the narcotics convention in line with the 2007 UN Declaration on the Rights of Indigenous Peoples.

La Paz has pledged zero tolerance for cocaine, which is distilled from coca, and supported alternative, licit coca-based products, including a pale green energy drink called Coca Brynco launched yesterday. "We want to reaffirm with this product that the coca leaf is healthy," said Nemesia Achacollo, the rural development minister. "We must defend our coca leaf and show it's not a drug."

Scientists have found that chewing coca is not harmful to health. It takes 200kg of coca, and a lot of chemicals, to produce 1kg of cocaine. Liberal advocacy groups such as the Washington Office on Latin America and the Transnational Institute, among others, have attacked the UN ban.

"The international community needs to get its priorities right and resist this culturally ignorant attempt to dictate to indigenous people in Bolivia," said Labour MP Jeremy Corbyn.

The European Union's stance remains unclear but Bolivian lobbying appears to have convinced Colombia and Macedonia to withdraw objections to its amendment, leaving the US potentially isolated.

A senior US official told the Associated Press a formal objection would be filed today to shore up the 1961 convention, which along with coca chewing prohibits 119 substances such as opium and herion. Bolivia's amendment would encourage other countries to pick the document apart for their own reasons, he said. "Over the long term [it] is not good for the planet's efforts to control and eventually solve the problem of drug abuse."