Newport City Council is reminding off-licences and pubs of their duty to check the ages of people buying alcohol.
They are also warning adults that it is an offence to buy alcohol, including beer, cider, liqueurs and alcopops, for anyone under the age of 18.
Trading standards and licensing teams are issuing the advice following an increase in reports from residents about groups of young people drinking in the streets and other public places.
A premises owner and the seller could be prosecuted if they make a sale to somebody under the age of 18. The owner also risks losing their licence.
Councillor Ray Truman, cabinet member for licensing and regulation, said: “The laws on selling alcohol to under-18s are there to protect them.
“Those selling alcohol to, and adults buying alcohol for, under-18s are putting those young people at risk in a number of ways. Not only can it have a serious impact on their lives, health and well-being, it can also make them vulnerable to harm from others.
“I urge premise owners, their employees and other adults to be responsible and encourage anyone with information about possible offences to report to the council’s trading standards team, the police or, anonymously, to Crimestoppers.
“You can do this if you notice young people hanging around shops asking adults to make purchases for them, know of anywhere that illegally sells alcohol to children or see young people drinking alcohol in parks or other public places.
“You will be helping to keep our young people and communities safe.”