Fierce fighting has been raging north-east of Kharkiv since a new Russian push across Ukraine’s northern border. Kharkiv, Ukraine’s second city, is just 30km (18 miles) from the border.
UK Foreign Secretary Lord Cameron has said it is up to Ukraine to decide how to use British weapons and insisted it has the right to strike targets on Russian territory.
Ukraine says North Korean missiles are being used inside Ukrainian territory, and Western intelligence agencies say Russia has been using Iranian-made drones in the conflict.
Russia launched a full-scale invasion of Ukraine in February 2022.
Mr Putin was speaking to foreign journalists at the annual St Petersburg International Economic Forum.
He also warned that the West was wrong to assume that Moscow would never use nuclear weapons.
“For some reason, the West believes that Russia will never use it,” Mr Putin said when asked by Reuters about the risk of nuclear escalation over Ukraine.
“We have a nuclear doctrine, look what it says. If someone’s actions threaten our sovereignty and territorial integrity, we consider it possible for us to use all means at our disposal.
“This should not be taken lightly, superficially.”
Mr Putin also dismissed the idea that Russia has plans to attack Nato territory.
“You should not make Russia out to be the enemy. You’re only hurting yourself with this, you know?” Mr Putin said.
“They thought that Russia wanted to attack Nato. Have you gone completely crazy? That is as thick as this table.