GWEN IFILL: The escalating crisis between Russia and Ukraine was a key topic as world leaders met today in Amsterdam.

Chief foreign affairs correspondent Margaret Warner has been traveling in Ukraine for the past week and has this report.

MARGARET WARNER: Rembrandt’s 17th century masterpiece “The Night Watch,” depicting Dutch militia of that time, served as the backdrop as President Obama warned of an unfolding 21st century threat.

PRESIDENT BARACK OBAMA: Europe and America are united in our support of the Ukrainian government and the Ukrainian people. And we are united in imposing a cost on Russia for its actions so far.

MARGARET WARNER: Mr. Obama and other G7 leaders met at The Hague to discuss nuclear security, but Russia’s seizure of Crimea from Ukraine overshadowed the talks.

Ukrainian marines in Crimea were blocked by Russian forces today and forced to abandon their ship in Donuzlav Bay. Russians also stormed three bases in recent days.

In response, Ukrainian forces started packing up and leaving Crimea, on orders from Kiev.

OLEKSANDR TURCHYNOV, Acting President, Ukraine (through interpreter): The National Security and Defense Council has made the decision to instruct the Ministry of Defense to conduct the redeployment of military units located in the autonomous Republic of Crimea.

MARGARET WARNER: NATO’s top commander, U.S. Air Force General Philip Breedlove, also voiced concerns about Russian troops near Eastern Ukraine, and what they mean for allied deployment in Europe.

GEN. PHILIP BREEDLOVE, Supreme Allied Commander, Europe, NATO: The force that is at the Ukrainian border now to the east is very, very sizable and very, very ready. You cannot defend against that if you are not there to defend against it.

MARGARET WARNER: Ukraine’s new foreign minister insisted the country’s troops are prepared to fight if Russia makes any new military moves.

ANDRIY DESHCHYTSYA, Foreign Minister, Ukraine: We are very much worried about this concentration of troops on our eastern borders, but at same time, we are ready to defend our homeland.

MARGARET WARNER: Meanwhile, Russia’s annexation of Crimea continued apace. At a Moscow ceremony, the flags of Crimea and Sevastopol’s navy base were installed in line with other Russian regional flags.

Prime Minister Dmitry Medvedev chaired a meeting on providing economic help to Crimea.

DMITRY MEDVEDEV, Prime Minister, Russia (through interpreter): The top priority task is to set up a system of governing the new territories. In Crimea, they have their own bodies of government and it is necessary to reformat them to match Russian standards within the shortest possible time.

MARGARET WARNER: Back in Crimea, schoolchildren returned from spring break to salute a new flag and anthem. And people started counting their money in Russian rubles.