The G7 group of top economic powers on Monday scrapped a planned meeting to be hosted by Russia as they sought to deepen Moscow's isolation over the Ukraine crisis.

After emergency talks on the Ukraine crisis called by U.S. President Barack Obama, the G7 said they would hold a meeting in Brussels without Russia instead of the wider G8 summit that was to have taken place in Sochi.

President Barack Obama said Monday that the United States and China can help ensure "respect for the sovereignty of nations" as he held talks on Ukraine with Chinese President Xi Jinping.

Obama said his first meeting with Xi this year, held on the sidelines of a major summit on nuclear security here, would deal with "a wide range of issues of mutual interest", including "the situation in Ukraine."

U.S. President Barack Obama arrived in the Netherlands on Monday to discuss with six other world leaders how to punish Russia for annexing Ukraine's Crimea region, including possibly excluding Moscow from the G8 club of rich nations.

Obama has called an emergency Group of Seven summit in The Hague to discuss what steps to take against Russia over Crimea, with Russian troops on Monday morning seizing another Ukrainian military base on the peninsula.

Ukraine on Monday ordered its outnumbered troops to withdraw from Crimea after Russia's lightning annexation of the peninsula as world leaders met for emergency talks on the worst East-West standoff since the Cold War.

The dramatic announcement came after Ukraine lost yet another base in Crimea and as world leaders gathered in The Hague for a security summit set to be dominated by concerns over the Ukraine crisis.

President Barack Obama opens a six-day trip to Europe and Saudi Arabia on Monday seeking to reassure U.S. allies amid the worst East-West crisis in years and to defend his nuclear diplomacy with Iran.

A visit to the third Nuclear Security Summit in The Hague -- which grew out of Obama's initiative to prevent the spread of weapons of mass destruction and radioactive material -- has been planned for months.

Prime Minister Tammam Salam on Friday received a phone call from U.S. president Barack Obama who congratulated him over the new cabinet's formation.

Obama stressed during the call on the importance of holding the presidential elections on time, before the constitutional deadline.

The White House on Friday billed President Barack Obama's Europe trip next week as a chance to show the United States leading a campaign to isolate Russia over its annexation of Crimea.

Obama will meet G7 leaders and confer with the leadership of European Union nations and NATO, as he seeks to impose costs on Moscow for its takeover of the Ukrainian region.

U.S. President Barack Obama meets Internet bosses on Friday to discuss his latest plans to overhaul U.S. spy agency surveillance practices which have infuriated the industry.

Obama was expected to meet company chiefs including Google executive chairman Eric Schmidt and Facebook founder Mark Zuckerberg who said last week he had called the president personally to express frustration with the vast online intelligence dragnets.

U.S. First Lady Michelle Obama took her daughters and mother to Beijing's former imperial palace Friday, on a China tour the White House emphasizes will be light on politics and heavy on personal diplomacy.

Obama, her daughters Malia and Sasha, and her mother Marian Robinson along with China's own first lady Peng Liyuan took a guided tour of the sprawling Forbidden City, waving to onlookers outside the central pavilion known as the Hall of Supreme Harmony.

U.S. President Barack Obama announced a new round of punitive measures for Moscow's annexation of Crimea on Thursday as Europe's leaders also readied to hit back at Russia with fresh sanctions.

President Vladimir Putin's spokesman condemned the new anti-Russian sanctions as unacceptable, and noted that some of the names on the blacklist caused "bewilderment."