High-level diplomatic efforts to resolve the crisis in Ukraine made little apparent headway at talks in Paris today.
Moscow and Washington are at odds and Russia's foreign minister refused to recognise his Ukrainian counterpart.
US Secretary of State John Kerry said discussions would continue in the coming days in an attempt to stabilise the crisis and he expected to meet Russian Foreign Minister Sergei Lavrov again in Rome tomorrow.
Russia had earlier rebuffed Western demands that its forces that have seized control of Ukraine's Crimea region should return to their bases.
NATO, at a meeting in Brussels, announced it was cutting back on co-operation with Russia to try to pressure it into backing down on Ukraine and suspended planning for a joint mission linked to Syrian chemical weapons.
The alliance said it would step up engagement with Ukraine's new leadership.
The European Union offered Ukraine's new pro-Western government €11bn in financial aid in the next couple of years provided Kiev reaches a deal with the International Monetary Fund. Germany, the EU's biggest economy, also promised bilateral financial help.
Russia and the West are locked in the most serious battle since the end of the Cold War for influence in Ukraine, a former Soviet republic with historic ties to Moscow that is a major commodities exporter and strategic link between East and West.
Ukraine pulled out of a trade deal with the EU under Russian pressure last year, sparking months of protests in Kiev and the 22 February ouster of Mr Yanukovych, a Russian ally.
Ukraine says Russia has occupied Crimea, where the Russian Black Sea fleet is based, provoking an international outcry.
Mr Lavrov said discussions on Ukraine would continue, but he did not talk to Ukrainian Foreign Minister Andriy Deshchitsya, whose new government is regarded as illegitimate by Moscow.
As he left the Foreign Ministry in Paris, Mr Lavrov was asked if he had met his Ukrainian counterpart. "Who is that?" the Russian minister asked.
Mr Deshchitsya said he believed a "positive outcome" would emerge.
Asked why he had not met Mr Lavrov, he shrugged his shoulders and raised his eyebrows.
A senior US State Department official denied Russian reports that Moscow and the Western powers had agreed that the Ukrainian government and opposition should stick to an EU-brokered peace deal.
Today's talks in Paris were an effort by France to capitalise on the presence of major power foreign ministers in the French capital for a long-scheduled meeting on Lebanon.
A first attempt to get Mr Lavrov and Mr Deshchitsya together at the Elysee Palace of President Francois Hollande failed, as did a subsequent attempt at the Foreign Ministry.
Meetings involving the foreign ministers of France, Russia, the US, Poland, Germany, and Ukraine took place in various combinations during the day, but never with the Russian and Ukrainian ministers in the same room together.
Later, President Barack Obama spoke by phone to British Prime Minister David Cameron and they expressed "grave concern over Russia's clear violation of Ukrainian sovereignty", the White House said.
Earlier, Mr Lavrov repeated Moscow's assertion - ridiculed by the West - that the troops that have seized control of the Black Sea peninsula are not under Russian command.
Asked whether Moscow would order forces in Crimea back to their bases, Mr Lavrov said: "If you mean the self-defence units created by the inhabitants of Crimea, we give them no orders, they take no orders from us."
A UN special envoy had to abandon a mission to Crimea after being stopped by armed men and besieged inside a cafe by a hostile crowd shouting "Russia! Russia!" Dutch diplomat Robert Serry flew to Istanbul after the incident.
In a sign of heightened tensions in eastern Ukraine, a pro-Russian crowd in Donetsk, Mr Yanukovych's home town, recaptured the regional administration building they had occupied before being ejected by police.
The West is pushing for Russia to return troops to barracks, accept international monitors in Crimea and Ukraine and negotiate a solution to the crisis through a "contact group" probably under the auspices of a pan-European security body.
Britain said it would join other European Union countries in freezing the assets of 18 Ukrainians suspected of misappropriating state funds, and Canada announced economic sanctions on senior members of the government of ousted President Viktor Yanukovych.
France said EU leaders meeting in Brussels tomorrow could decide on sanctions against Russia if there is no "de-escalation" by then.
Other EU countries, including Germany, are more reticent about sanctions.
President Vladimir Putin yesterday defended Russia's actions in Crimea, which used to be Russian territory, and said he would use force only as a last resort.
The Pentagon will more than double the number of US fighter jets on a NATO air patrol mission in the Baltics and do more training with Poland's air force as it strives to reassure allies alarmed by the crisis in Ukraine, officials in Washington said.