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."

"I believe ultimately that by working together, China and the United States can help strengthen international law, respect for the sovereignty of nations and establish the kind of rules, internationally, that allow all people to thrive," said the US leader.

The United States and the EU, which have imposed sanctions on Russian officials, argue that Russia violated Ukraine's sovereignty by backing an independence referendum in Crimea that led to the Black Sea peninsula's annexation by Moscow.

Sitting alongside Obama, Xi did not address the subject of Ukraine specifically but said there was "greater space where China and the United States are cooperating" and agreed with the US president that Beijing and Washington should forge a new "major power relationship."

Analysts say Xi is unlikely to spell out China's position on the Ukraine crisis on his European trip but Chinese officials have reiterated calls for "calm and restraint".

Xi's four-country trip comes after China lodged a rare abstention on a Western-backed UN Security Council resolution condemning Crimea's referendum on joining Russia, rather than vetoing it alongside Moscow.

The two leaders are in the Hague for a meeting of the Nuclear Security Summit (NSS), ostensibly to push forward efforts to prevent dangerous nuclear material getting into the wrong hands, but overshadowed by the Ukraine crisis.

Obama has called an emergency meeting of the group of seven (G7) top industrial nations to discuss the unrest in Crimea, which has plunged relations between Russia and the West to lows not seen since the end of the Cold War.

The meeting was expected to take place later Monday and could see Russia expelled from the wider G8 as a punishment for its annexation of Crimea.