US President Joe Biden said Thursday he had called an emergency NATO summit to ensure international sanctions on Russia over its invasion of Ukraine are kept in place over the long term.

"The maintenance of sanctions... (is) increasing the pain.... Why I asked for this NATO meeting today, is to be sure that after a month we will sustain what we're doing -- not just next month, the following month -- but for the remainder of this entire year," he told a news conference at NATO headquarters.

"That's what will stop him," he said, referring to Russian President Vladimir Putin.

Biden insisted that he had never wielded the threat of sanctions as a form of deterrence, to head off Putin's ordered invasion of Ukraine.

"Let's get something straight. You remember, if you covered me from the very beginning, I did not say that, in fact, the sanctions would deter him. Sanctions never deter. You keep talking about that! Sanctions never deter," he told journalists.

The United States, the European Union and their allies have unleashed several coordinated waves of sanctions on Russia since its February 24 invasion of Ukraine.

The punishment has severely hit Russia's finances and economy, and also target Russian defence companies and oligarchs, as well as Putin and his closest ministers and aides.

Washington and Brussels have warned of further sanctions on Moscow, to increase the cost to it of keeping up its war.