Russian President Vladimir Putin has frequently talked about using nuclear weapons in recent years, and he's done so again at an economic forum in St. Petersburg.

Putin was responding to a moderator's questions on Friday after delivering a speech.

He said nuclear weapons could be used if Russia's sovereignty and territorial integrity come under threat.

He added that he does not think such a scenario has occurred, but does not rule out changing the country's nuclear doctrine.

Washington recently gave Ukraine permission to use US-made weapons for limited strikes on Russian territory. In St. Petersburg, Putin said his country reserves the right to supply long-range weapons to adversaries of the West.

In his speech, Putin said Russia remains a key part of global trade, and maintains economic ties with friendly nations despite all obstacles and sanctions.