The European Union is set to unveil a fresh package of sanctions against Russia as western leaders staged a show of support for the beleaguered Ukrainian government.
The move comes after David Cameron, US president Barack Obama and other senior Nato leaders held talks with Ukrainian president Petro Poroshenko at the alliance summit in Wales.
British government sources said that the EU would unveil measures in Brussels tomorrow - including sanctions targeting state-owned Russian defence and energy companies.
They will also look to strengthen existing measures against state-owned banks and extend controls on the sale of military equipment to cover dual-use civilian military kit.
It is expected that the United States and Canada will announce similar measures in a co-ordinated move against Moscow.
One source said they were designed to force Mr Putin "to the negotiating table and off the battlefield" in eastern Ukraine.
The move came as the Ukraine's ambassador to the United Nations said a convoy of tanks and armoured vehicles seized by pro-Russian separatists was moving towards the key Black Sea port of Mariupol.
Yuriy Sergeyev told a news conference in New York that the column had been halted by Ukrainian forces and civilians, adding "The fight is still going on right now".
Earlier, Mr Cameron said that existing sanctions were already hurting the Russian economy.
"It is making a difference. The Russian economy was growing, it is now shrinking. Russian banks - some of them are getting short of money. The Russian stock market, the rouble, have suffered," he told BBC1 Breakfast.
"What Russia needs to understand is that if they continue with this approach in Ukraine, this pressure will be ramped up.
"The Ukrainians know that they have our support and this sanctions pressure is the right way to tell the Russians that what they are doing is unacceptable."
Mr Poroshenko said the summit represented a "landmark event" in the relationship between his country and Nato.
Speaking at a press conference alongside Nato secretary general Anders Fogh Rasmussen, he said: "I have never felt such a strong, enormous support from all the leaders of the countries, heads of government and representatives of the people of Nato member states as was demonstrated today for Ukraine.
"It is definitely a landmark event of the highest level in the 20-year history of the partnership between Ukraine and Nato.
Mr Poroshenko said Nato member states had agreed "bilateral military technical co-operation on non-lethal and lethal items" while Ukraine had agreed to work towards full inter-operability between its forces and those of the trans-Atlantic alliance, including by taking part in joint exercises.
"The completely new security situation created by this aggression calls for our joint action to counter the emergent security challenges, including hybrid warfare threats and thus shape a new strategic framework of Nato-Ukraine co-operation," he said.