US president Barack Obama arrived in the Netherlands for four days of meetings with European allies in a visit likely to be dominated by the crisis in Ukraine.
An emergency meeting of G7 leaders, convened by Mr Obama last week, will take place on the fringes of a Nuclear Security Summit scheduled for today and tomorrow.
More than 50 nations, including Russia and Ukraine, are participating in the event, with Russia expected to be represented by its foreign minister, Sergei Lavrov.
Mr Obama will visit Nato headquarters in Brussels and meet the head of the European Commission and Council on Wednesday, before travelling to Rome where he is scheduled to meet Pope Francis.
His visit comes as a senior Nato official raised concerns about the build-up of Russian troops at Russia’s border with Ukraine.
In Brussels yesterday, Nato's supreme allied commander for Europe, Gen Philip Breedlove, said Nato was concerned in particular about Moldova's breakaway region of Transdniester, which has already requested to join the Russian federation.
The narrow strip of land broke away from Moldova in 1990 but is not recognised by the international community.
'Very sizeable' force
"The Russian force that is at the Ukrainian border now to the east is very, very sizeable, and very, very ready," the US air force general said at a conference held by Brussels-based think tank the German Marshall Fund.
The situation in Ukraine will be discussed today by leaders of the world's largest economies, including British prime minister David Cameron and Chinese president Xi Jinping.
The Chinese leader arrived for his first official visit to Europe on Saturday, and is due to visit Brussels next week.
The meeting between Mr Xi and western leaders, including Mr Obama, could be strategically significant, as the West tries to consolidate a strong position on Russia’s incursion into Ukraine.
China, which has supported Russia's position during the Syrian conflict, has remained relatively silent on the Ukraine issue. It abstained from voting on a UN Security Council resolution declaring the Crimean referendum invalid just over a week ago. The resolution was vetoed by Russia.
Under pressure
Mr Cameron has come under pressure from opposition parties to suspend Russia from the G8 when the G7 meets today.
On Friday the EU expanded its list of Russian individuals who are to be targeted by travel bans and asset freezes. It extended the restrictions to 12 individuals, including Russian deputy prime minister Dmitry Rogozin and the head of the Rossiya Segodnya state news agency.
While the EU added 12 names to the list of 21 individuals announced a week ago, EU leaders refrained from committing to broader – so-called “phase three” – sanctions. However, the European Commission has begun preparatory work on what deeper economic sanctions would involve.
The US has already imposed its own targeted sanctions, including restrictions on the Russian Bank Rossiya, which the White House says is used by senior Russian officials to channel money.
The western sanctions prompted a response from Russia, which announced its own sanctions against US officials and members of Congress, including House of Representatives speaker John Boehner, Senate Democratic majority leader Harry Reid and senior Republican senator John McCain.
