Russian aggression in Ukraine has only reinforced the unity of the US and Europe, Barack Obama has declared.
The US president and German Chancellor Angela Merkel have been holding talks about the prospects of reviving an elusive peace plan to end the conflict.
Their talks came as Germany and France prepared to meet this week with Russian and Ukrainian leaders.
Without a resolution, Mr Obama said, "Russia's isolation will only worsen, both politically and economically".
During a joint news conference at the White House, Mrs Merkel said it was crucial that the West stand up for Ukraine.
"If we give up this principle of territorial integrity of countries, then we will not be able to maintain the peaceful order of Europe," she said.
Both the US and Germany have emphasised a need to resolve the conflict through diplomacy. However, a recent surge in violence has led the White House to reconsider its opposition to sending lethal aid to Ukraine.
Mrs Merkel opposes the West arming Ukraine's beleaguered military.
Earlier, the European Union decided to temporarily hold off imposing sanctions on more Ukrainian separatists and Russians pending the outcome of possible peace talks this week.
With the leaders of Germany, France, Russia and Ukraine due to hold negotiations in Minsk, Belarus, on Wednesday, the EU has delayed placing asset freezes and travel bans on 19 more individuals, including five Russians, for their actions linked to eastern Ukraine.
"The principle of the sanctions is maintained but the application will depend on what happens on the ground," French foreign minister Laurent Fabius said.
Mr Fabius said political advisers would meet on Tuesday in Minsk to "hopefully" set up talks for the following day between Mrs Merkel, French President Francois Hollande, Russian President Vladimir Putin and Ukrainian President Petro Poroshenko.
"There is plenty of work left to do. There are a series of questions to answer," he said, including where heavy weapons should be withdrawn and how to secure Ukraine's borders.
The EU foreign ministers are backing fresh diplomatic efforts to end the conflict in Ukraine, many rejecting calls by some US politicians to provide lethal defensive weapons to Kiev.
German foreign minister Frank-Walter Steinmeier said the suggestion of sending weapons did not come up in EU talks today but if a ceasefire cannot be agreed upon in Minsk - and respected - it would lead to an escalation of the conflict in eastern Ukraine.
The EU has visa bans and asset freezes on 132 individuals and 28 entities for their actions in eastern Ukraine. Nine more entities - companies and separatist groups - could also be added to the list.
EU foreign policy chief Federica Mogherini said Mr Poroshenko himself supported the EU delay to give time for the peace talks to take place.
As diplomatic efforts to find a resolution to the Ukraine crisis were stepped up, a powerful explosion rocked a chemical plant and set it on fire outside the separatist stronghold of Donetsk in eastern Ukraine.
No casualties or damage were immediately reported from the huge blast at the plant, which the rebels said was hit by government shelling. The city lies in the middle of Ukraine's industrial heartland and explosions like this have long been feared.
The rebel Donetsk News Agency said the chemical plant west of the city was hit by a Ukrainian artillery shell but the fire was put out and it posed no danger to residents.