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President Obama made a series of phone calls on Saturday to world leaders about the ongoing crisis in Ukraine, including Prime Minister David Cameron, French President Francois Hollande, and Italian Prime Minister Matteo Renzi, the White House said.
He also held a conference call about the situation with the presidents of Lithuania, Latvia and Estonia, White House spokesman Josh Earnest said in a statement.
The recent spell of warm weather is set to continue tomorrow - with top temperatures in the capital set to beat holiday spots in Spain and the South of France.
Highs are expected to reach 18C in London tomorrow - whereas Majorca will see around 17C and luxury hotspot St Tropez just 15C.
St James' Park saw the hottest temperature of the year yesterday at 16.9C.
Overnight tonight warm winds from the south will keep temperatures mild, with clear skies expected all day tomorrow across the capital.
French President Francois Hollande has said he agrees with President Obama that Russia must take steps toward pulling out troops sent into Ukraine's Crimean peninsula or face worse relations with the world community.
An official statement said the two men spoke by phone today and agreed that a planned March 16 referendum in Crimea - home to many pro-Russian citizens -"lacks any legal basis".
The statement said that unspecified "new measures" would be taken unless progress was made on such issues.
The Deputy Prime Minister has said he would be concerned about any move which would "lessen the signal" that people should pay the TV licensing fee, after plans revealed that the Government was considering easing pressure on courts.
He said: "We will look at it but I think you need to be aware that if you have a 1% drop in the payment of the licence fee, that would lose you money which at the moment would cover the money for 10 local radio stations.
"So it's important that people do pay their licence fee and I don't want to see any relaxation of that because I'm a great fan of the BBC. I think a lot of people rely on the BBC locally and nationally for their news and for a lot else besides.
A Ukrainian border patrol plane came under fire while flying at about 1,000 metres (3000 feet) near the administrative border with Russian-occupied Crimea, the border guards said.
No one was hurt when gunmen opened fire on the unarmed aircraft, a spokesman said. The Diamond light aircraft was flying three crew on an observation mission, the spokesman said.
A convoy of hundreds of Russian soldiers in 65 troops trucks drove into a base near Crimea's capital Simferopol, according to Reuters.
The convoy was accompanied by eight armoured vehicles, two ambulances, petrol tankers and other hardware. Russia has said its only troops in Crimea are those normally stationed there with its Black Sea Fleet, an assertion Washington calls "(President Vladimir) Putin's fiction".
Kiev said there are 30,000 Russians in Crimea. The Pentagon estimates their number at around 20,000.
A cat has been reunited with its owner after going missing for eight years.
Jesse was 18 months old when he bolted out of his cat box on the way to the vets near Mow Cop on the Staffordshire-Cheshire border in 2006.
Owner Leslie Corbett put up missing cat posters and searched the area for days but eventually gave up hope of a return.
But last month the vet telephoned Mrs Corbett to say that Jesse, who had been microchipped, was found "hanging around" nearby.
"We have absolutely no idea where he's been," she told the BBC. "All we know is he's happy and healthy."
A 30-year-old man has been arrested in connection with an arson attack on a mosque.
Police were called to the mosque in Manor Road, Bletchley, Milton Keynes, before 1am today after reports of a man seen acting suspiciously before a fire was started at the rear of the building.
The man was later arrested on suspicion of arson with intent to endanger life and remains in police custody.
ITV News Europe correspondent Emma Murphy reports from Donetsk in Ukraine:
Vitali Klitschko to visit #Donetsk tomorrow. Kiev trying to address feeling of isolation in East?