David Cameron has issued a fresh warning to Russia that it faces international isolation if it continues to interfere in Ukraine.
Arriving tonight in Brussels for a hastily arranged summit of the G7 group of industrialised nations, the Prime Minister said Moscow needed to engage with the Ukrainian leadership to find a diplomatic solution to the crisis.
Mr Cameron is due deliver his message personally to Vladimir Putin when he meets the Russian president in Paris tomorrow for face-to-face takes ahead of the D-Day 70th anniversary commemorations on Friday.
"The G7 should send a clear message of support to Ukraine and a united message to President Putin that he needs to engage with the Ukrainian government to find a diplomatic solution to the crisis," Mr Cameron said.
"That's what I'll be saying to President Putin myself tomorrow. This is the first summit without Russia since the 1990s. And until they change course, they need to understand that they will face continuing isolation and no seat at the table."
Mr Putin is expected to meet other European leaders during the course of the next two days - when he will hear a similar message - although US president Barack Obama is thought to be unlikely to see him.
The G7 leaders effectively expelled Russia from the annual gathering of the top industrialised nations, which Mr Putin had been due to to host in Sochi, following its annexation of Crimea.
Although European leaders and Mr Obama have said they stand ready to impose tougher sanctions on Moscow, a fresh wave of penalties is not expected to be announced.
Leaders will focus on how to support new Ukrainian president-elect Petro Poroshenko and tackle long-standing problems with corruption in Ukraine as well as its ailing economy.
It will be the first G7 since the summit staged in Lyon, France in 1996.
Earlier, Mr Obama - in Warsaw to celebrate 25 years of Polish democracy - declared his support for the right of Ukraine to determine its own destiny free from Russian interference.
"Our free nations will stand united so that further Russian provocations will only mean more isolation and costs for Russia," he said.
"Because after investing so much blood and treasure to bring Europe together, how can we allow the dark tactics of the 20th century to define this new century?"
Mr Obama, who held his first extended talks with Mr Poroshenko, announced the first direct US military assistance for Kiev's armed forces with five million dollars (£3 million) worth of equipment, including body armour and night vision goggles.
"Bigger nations must not be allowed to bully the small, or impose their will at the barrel of a gun or with masked men taking over buildings," he said.
On the ground in Ukraine, pro-Russian separatist rebels were reported to have taken two military bases in the eastern region of Luhansk.
A border guard base, which was said to have fallen to the rebels after days of fierce combat, and a national guard base that was seized in an attack which began on Tuesday.
Fighting was reported to be continuing near the towns of Krasny Liman and Sloviansk in neighbouring Donetsk region.
In a statement tonight, the G7 leaders condemned Russia's "unacceptable interference" in Ukraine's sovereign affairs and warned that they were ready to impose wide-ranging economic sanctions "should events so require".
"We are united in condemning the Russian Federation's continuing violation of the sovereignty and territorial integrity of Ukraine," they said.
"We stand ready to intensify targeted sanctions and to implement significant additional restrictive measures to impose further costs on Russia should events so require."