British Prime Minister Boris Johnson has described the invasion of Ukraine by Russia as a "macho war" and stated that Vladimir Putin would not have started the war if he were a woman.

"If Putin was a woman, which he obviously isn't, if he were, I really don't think he would have embarked on a crazy, macho war of invasion and violence in the way that he has," Johnson told German TV broadcaster ZDF, following the G7 summit in Schloss Elmau.

"If you want a perfect example of toxic masculinity, it's what he is doing in Ukraine."

He also said the world needs "more women in positions of power".

Johnson's comments come after the G7 leaders mocked Putin's penchant for bare-chested horseback riding photographs of himself.

As they sat down for a meeting, Johnson reportedly asked the other leaders if they should all take their jackets off.

G7 leaders met in the Bavarian Alps this week. Photo / Getty Images

"We all have to show that we're tougher than Putin," Johnson said. "Show them our pecs," he added.

Justin Trudeau chimed in on the joke, suggesting they go "bare-chested horseback riding".

"Oh yes," replied European Commission president Ursula von der Leyen. "Horseback riding is the best".

Over the years, multiple photos of Putin released by Russian state media show the Russian leader bare-chested, riding horses or carrying hunting rifles. He has also released images sunbathing topless as well as riding a submersible in the Baltic Sea.

Russian state media photo of Putin in Siberia in 2019. Photo / Getty Images

Leaders of the world's biggest developed economies said Tuesday they would explore far-reaching steps to cap Russia's income from oil sales that are financing its invasion of Ukraine and struck a united stance to support Kyiv for "as long as it takes" as the war grinds on.

The final statement from the Group of Seven summit in Germany underlined their intent to impose "severe and immediate economic costs" on Russia.

It left out key details on how fossil fuel price caps would work in practice, setting up more discussion in the weeks ahead to "explore ... the feasibility" of measures to bar imports of Russian oil above a certain level.

That would hit a key Russian source of income and, in theory, help relieve the energy price spikes and inflation afflicting the global economy as a result of the war.

"We remain steadfast in our commitment to our unprecedented coordination on sanctions for as long as necessary, acting in unison at every stage," the leaders said.

The G-7 leaders — representing the US, Germany, France, Italy, the UK, Canada and Japan — on Monday pledged to support Ukraine "for as long as it takes" after conferring by video link with Ukrainian President Volodymyr Zelenskyy. Zelenskyy has openly worried that the West has become fatigued by the cost of a war that is contributing to soaring energy costs and price hikes on essential goods around the globe. The G7 has sought to assuage those concerns.

Leaders also agreed on a ban on imports of Russian gold and to step up aid to countries hit with food shortages by the blockade on Ukraine grain shipments through the Black Sea.

"We agree that (Russian) President (Vladimir) Putin must not win this war, and we will continue to keep up and drive higher the economic and political costs for President Putin and his regime," said the summit host, German Chancellor Olaf Scholz. "For that, it is important to stand together - including in the long haul that we certainly still face."

British Prime Minister Boris Johnson (left), US President Joe Biden, German Chancellor Olaf Scholz and French President Emmanuel Macron. Photo / AP

French President Emmanuel Macron said Russia "cannot and should not win" the war in Ukraine — as its terrible toll was on full view the day after a Russian missile strike hit a shopping mall in the town of Kremenchuk, killing 18 people.

The price cap — pushed by US President Joe Biden — would in theory work by barring service providers such as shippers or insurers from dealing with oil priced above a fixed level. That could work because the service providers are mostly located in the European Union or the UK and thus within reach of sanctions. To be effective, however, it would have to involve as many consuming countries as possible, in particular India, where refiners have been snapping up cheap Russian oil shunned by Western traders. Details on how the proposal would be implemented were left for continuing talks in coming weeks.

The US has already blocked Russian oil imports, which were small in any case. The European Union has decided to impose a ban on the 90 per cent of Russian oil that comes by sea, but the ban does not take effect until the end of the year, meaning Europe continues to send money to Russia for energy even while condemning the war. Meanwhile, higher global oil prices have softened the blow to Russia's income, even as Western traders shun Russian oil.

From the secluded Schloss Elmau hotel in the Bavarian Alps, the G7 leaders will move to Madrid for a summit of Nato leaders, where fallout from Russia's invasion of Ukraine will again dominate the agenda.

All G7 members other than Japan are Nato members, and Japanese Prime Minister Fumio Kishida has been invited to Madrid.

- Additional reporting by AP