The US President, Barack Obama, has issued a warning to Russia’s Vladimir Putin to keep his troops out of Ukraine or face even tougher sanctions, calling Russia a “regional power” which would struggle to compete with America’s global influence.

So far, Washington and the European Union (EU) have responded to President Putin’s annexation of Crimea this month with sanctions on dozens of individuals, most of them directly linked to Mr Putin’s actions in Crimea after his ally Viktor Yanukovych was ousted by protesters in Kiev.

Those diplomatic moves have so far had little effect in Moscow, and Western leaders have faced criticism that they are not being tough enough in the face of a more muscular Russia.

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Mr Obama appeared to meet such criticism by making clear that the US is “the most powerful nation in the world” and one that other countries looked to in order to take the lead on global crises such as the conflict in Syria. Asked at a nuclear-security conference in The Hague whether Russia was now Washington’s top geopolitical foe, he replied: “America’s got a whole lot of challenges.” But Russia, he added, “is a regional power that is threatening some of its immediate neighbours, not out of strength but out of weakness. We [the US] have considerable influence on our neighbours. We generally don’t need to invade them in order to have a strong co-operative relationship with them.”

Shape Created with Sketch. In pictures: Ukraine crisis Show all 12 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. In pictures: Ukraine crisis 1/12 Ukraine crisis People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea. 2/12 Ukraine crisis In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea. 3/12 Ukraine crisis Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea. 4/12 Ukraine crisis Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol. 5/12 Ukraine crisis Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II. 6/12 Ukraine crisis A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise. 7/12 Ukraine crisis An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol, 8/12 Ukraine crisis 9/12 Ukraine crisis Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th. 10/12 Ukraine crisis An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th. 11/12 Ukraine crisis Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya. 12/12 Ukraine crisis Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting. 1/12 Ukraine crisis People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea. 2/12 Ukraine crisis In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea. 3/12 Ukraine crisis Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea. 4/12 Ukraine crisis Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol. 5/12 Ukraine crisis Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II. 6/12 Ukraine crisis A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise. 7/12 Ukraine crisis An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol, 8/12 Ukraine crisis 9/12 Ukraine crisis Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th. 10/12 Ukraine crisis An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th. 11/12 Ukraine crisis Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya. 12/12 Ukraine crisis Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting.

Both Washington and the EU have recently intensified their warnings that they will not hesitate to impose tougher and broader economic sanctions if Russia does not de-escalate the crisis in Ukraine or makes any attempt to move its troops beyond Crimea and into eastern or western Ukraine.

The Nato Secretary General, Anders Fogh Rasmussen, said he was “ very much concerned” about a build-up of Russian troops along Ukraine’s borders. Mr Obama said that Russia “has a right legally to have troops on its own soil” but warned against any further action.

“We also are concerned about further encroachment by Russia into Ukraine,” Mr Obama said. “I think that will be a bad choice for President Putin to make.”

If the situation deteriorated, Mr Obama said the US would be ready and willing to impose sanctions on sectors including energy, arms, financial services and trade, even if there would be an impact on the rest of the world. Europe has close economic ties to Russia, and its punitive measures against Russia have been tempered by caution over potential knock-on effects of sanctions.

Mr Obama stressed that any sanctions “will have the greatest impact on Russia”, a sentiment echoed by the Dutch Prime Minister, Mark Rutte, who was hosting the nuclear-security summit.

“Obviously, you can never guarantee that the people in Europe, in Canada, in the US, would not be hurt,” Mr Rutte said. “But obviously, we will make sure that we will design these sanctions in such a way that they will have maximum impact on the Russian economy and not on the European, the Canadian, the Japanese or the American economy.”

On Monday, the leaders of the US, the UK, France, Italy, Germany, Japan and Canada suspended Russia from the G8 group of nations and cancelled a meeting of the group in the Russian city of Sochi in June. They will instead meet as G7 in Brussels.

While the Russian Foreign Minister, Sergei Lavrov, was initially dismissive of the move, the Kremlin said it was keen to maintain diplomatic contacts. “The Russian side continues to be ready to have such contacts at all levels, including the top level,” Mr Putin’s spokesman, Dmitry Peskov, told Interfax news agency.