THE HAGUE — As Russia consolidated its hold on Crimea, raising its flag over seized military bases and detaining ousted Ukrainian commanders on Sunday, President Obama and his international allies prepared to meet here in an effort to develop a strong, united response despite their diverging interests in dealing with the Kremlin.

After Russia’s invasion of Crimea and the lightning annexation of the peninsula by President Vladimir V. Putin last week, Mr. Obama’s decision to convene the leaders of several European countries, along with Canada and Japan, brought the nations — once again the Group of 7, without Russia — together for the first time since the crisis in Ukraine upended the stability and security of Europe.

Susan E. Rice, Mr. Obama’s national security adviser, acknowledged that the president’s weeklong trip, including a meeting with Pope Francis on Thursday and a stop in Saudi Arabia on Friday, would be overshadowed by Ukraine and the need to press for Western unity. She expressed confidence that the meeting here on Monday would “deepen” coordination.

But as the United States ratchets up economic sanctions against Russia, it may be difficult for Mr. Obama to bring along European allies, who are more economically intertwined with Russia and ended their own summit meeting Friday with no detailed mention of tougher sanctions.