The United States and its G7 allies rolled out new sanctions against Russia's "war machine" today, targeting Moscow's lucrative diamond trade and more entities linked to the invasion of Ukraine.

Leaders from the Group of Seven wealthy democracies are meeting in Hiroshima, Japan, with Russia's $4-5 billion annual trade in diamonds in the crosshairs.

Vladimir Putin's invasion of Ukraine 15 months ago has prompted waves of sanctions that have helped plunge his country into recession and drained the Kremlin's war chest.

The G7 is now looking to tighten the screws further, strengthening existing sanctions, closing loopholes and subjecting more Russian firms and their international partners to punitive restrictions.

Ukrainian President Volodymyr Zelensky is expected to address the summit over the weekend by video link, with hosts Japan tamping down speculation he could make a last-minute, in-person appearance.

Washington got the ball rolling on more sanctions early today, with a senior US administration official saying 70 more entities from Russia and "other countries" would be placed on a US blacklist.

"And there will be upwards of 300 new sanctions against individuals, entities, vessels and aircraft," the official said.

As the G7 weighs how to collectively choke Russia's trade in diamonds -- including high-tech methods of tracing -- Britain announced its own "ban on Russian diamonds".

London said it was also targeting imports of aluminium, copper and nickel.

"As today's sanctions announcements demonstrate, the G7 remains unified in the face of the threat from Russia and steadfast in our support for Ukraine," said Prime Minister Rishi Sunak.

The G7 is likely to stop short of an outright ban on Russian diamonds, at least for now. But according to officials, the summit will signal a determination to act.

"Russian diamonds are not forever", said EU Council President Charles Michel. "We will restrict trade in Russian diamonds."

EU member state Belgium is among the largest wholesale buyers of Russian diamonds, along with India and the United Arab Emirates.

The United States is a major end-market for the finished product.

Economists are divided about just how much G7 and other sanctions have hurt the Russian war effort.

The Russian economy contracted 2.1 percent in 2022, a trend that continued early this year.

But Moscow has adapted quickly, introducing strict capital controls, diverting trade to allies like China and reportedly borrowing evasion techniques from much-sanctioned countries like Cuba, Iran and North Korea.

The International Monetary Fund has projected a modest 0.7 percent economic rebound in 2023.

G7 leaders will also use the summit to woo invited leaders from India and Brazil, two regional powers that have sometimes been reluctant to criticise Moscow, or China.

Michel said military support for Ukraine, which is believed to be preparing a long-anticipated spring offensive, would also be discussed among G7 members today, along with training for fighter pilots.

"We will assess the level of additional support that will be needed. It's very clear that Ukraine needs more military equipment," he added.