Neither US President Donald Trump nor Russian President Vladimir Putin are attending the G20.
The widely leaked US peace plan includes proposals that Kyiv had previously ruled out: ceding eastern areas it now controls, significantly cutting its army size, and pledging not to join Nato.
Washington has been pressing Kyiv to accept, and sent senior Pentagon officials to the Ukraine earlier this week to discuss the proposals.
But there is nervousness in European capitals over what has been perceived as a set of terms heavily slanted in Moscow's favour, with EU foreign policy chief Kaja Kallas calling the prospect of it being adopted a "very dangerous moment".
According to Reuters news agency, she told reporters: "We all want this war to end, but how it ends matters. Russia has no legal right whatsoever to any concessions from the country it invaded; ultimately the terms of any agreement are for Ukraine to decide."
Ahead of the Ukraine talks at the G20, Sir Keir said gathered leaders would "discuss the current proposal on the table, and in support of President Trump's push for peace, look at how we can strengthen this plan for the next phase of negotiations".
He continued: "Not a day has passed in this war where Ukraine hasn't called for Russia to end its illegal invasion, roll back its tanks and lay down its guns.
"Ukraine has been ready to negotiate for months, while Russia has stalled and continued its murderous rampage.
"That is why we must all work together, with both the US and Ukraine, to secure a just and lasting peace once and for all."
As part of the White House's plan, Ukraine would be obliged to cut the size of its army and pledge not to join the Nato military alliance, a long-held Kremlin demand.
Speaking on Friday, Trump warned that Ukraine would lose more territory to Russia "in a short amount of time" and that Zelensky "is going to have to approve" the plan.
Trump said he had given Ukraine until Thursday to agree to the plan, which he described as an "appropriate" deadline, though added it could be pushed back if talks were "going well".
Ukraine relies on deliveries of US-manufactured advances weaponry to arms its forces, including air defence systems.
Kyiv has also been dependent on intelligence provided by Washington since Russia launched its full-scale invasion of Ukraine in 2022.
During a meeting with his security cabinet on Friday, Putin confirmed the US had presented its proposed peace plan, and said it could be the "basis" for a settlement - though added detailed talks on its terms had not yet been held in the Kremlin.
He said Russia was willing to "show flexibility" but was also prepared to fight on, despite reported heavy Russian combat casualties.
"We have our tasks, our goals," Putin told his military commanders earlier this week, adding: "The chief one is the unconditional achievement of the aims of the special military operation [full-scale war]."