The West should consider ways to stop Russia using its gold reserves to prop up the rouble, the prime minister said.
Boris Johnson's comments came as the UK announced sanctions on 65 more groups and individuals, including a private military firm and a major Russian bank.
Speaking ahead of a Nato summit, the PM said the UK and the West must "tighten the economic vice" on Vladimir Putin.
He said Russia crossed "red lines" by targeting Ukrainian civilians and tough steps now could help shorten the war.
In addition to the new raft of financial sanctions, the UK government has announced plans to send 6,000 more missiles to Ukraine.
Leaders from Nato, the EU, and the G7 are holding emergency meetings in Brussels to discuss the ongoing situation in Ukraine.
On Thursday, Nato announced the creation of new battle groups in Bulgaria, Hungary, Romania, and Slovakia, while Ukrainian President Volodymyr Zelensky used a virtual address to urge the bloc to increase its supply of military equipment.
Ahead of the talks, Mr Johnson said that, by deliberately bombing civilian centres, Putin had "already crossed the red line into barbarism".