* Tensions between Russia and Ukraine rise
* Temperatures to drop towards seasonal norm for next 15 days
* Several LNG tanker arrivals help meet higher demand (Re-leads on rising Ukraine tension, updates prices)
LONDON, April 7 British wholesale prices for natural gas rose sharply on Monday afternoon on fears Moscow could take over eastern Ukraine, a vital entry point for Russian supplies piped into Europe.
Gas for delivery the next day rose 5 percent from Friday's levels to 51 pence per therm as concerns over a possible Russian supply cut to Ukraine re-emerged. Gas prices for delivery next winter were also up 5 percent, at 63.10 pence a therm.
Ukraine is one of Europe's key gas supply hubs. The former Soviet republic imports around half of its gas needs from Russia, which also meets a third of the European Union's demand. Forty percent of EU-bound Russian gas flows through Ukraine.
"We've seen some pretty alarming news about Ukraine today, so the market has begun to price a risk premium into gas again," one gas trader said.
Pro-Russia activists occupying a regional government building in Donetsk, in eastern Ukraine, proclaimed the creation of a separatist Donetsk republic, prompting Ukraine's interim president to say Moscow had entered a "second stage" of operations aimed at breaking up Ukraine.
Also on Monday, the U.S. ambassador to the Organisation for Security and Cooperation in Europe said Russia had amassed tens of thousands of troops near the border with Ukraine, calling on Moscow to take steps to de-escalate the situation.
Gas prices had already risen earlier in the day on expectations of colder weather.
"We're expecting somewhat colder weather for the next two weeks to lift heating demand, so that's led to some bidding up of gas prices this morning," another gas trader said.
Meteorologists said they expected average British temperatures to drop from above 10 degrees Celsius on Monday towards the seasonal norm of 7.5 degrees by Tuesday, where they were set to remain for the next 15 days.
Despite the higher demand and reduced flows from Norway , Britain's gas system was balanced on Monday, according to National Grid data, with demand of 206.3 million cubic metres (mcm) to be met by flows of 209 mcm.
A sharper rise in gas prices had been prevented by the scheduled arrival of several liquefied natural gas (LNG) tankers in Britain over the next two weeks, traders said.
Almost a million cubic metres of LNG is expected to arrive between Monday and April 15, spread over four vessels.
Once fed into the grid, these ships carry enough gas to meet more than two and a half days of current British demand.
In Britain's power markets, EDF Energy's 500-megawatt Hunterston B-7 nuclear unit returned to the grid over the weekend, helping to cushion the expected increase in demand.
Power prices for baseload (24 hours) delivery on Tuesday had fallen 60 pence ($1) per megawatt-hour (MWh) since opening to 39 pounds a MWh. GAS UK natural gas prices <0#TRGBNBP:> National Grid instant flows <0#NGRID-FLOW-RT> National Grid gas demand <0#NGRID-DSR-DEM> North Sea gas and power maintenance
POWER UK: baseload prices <0#TRGBB:>, peakload prices <0#TRGBP:>, outages FRANCE: baseload prices <0#TRFRB:>, peakload prices <0#TRFRP:>, outages, report GERMANY: baseload prices <0#TRDEB:>, peakload prices <0#TRDEP:>, outages, report CENTRAL EUROPE: report SWITZERLAND: outages, reservoir levels NORDIC: report EU CARBON PRICES SPEEDGUIDE:
($1 = 0.6028 British pounds) (Reporting by Henning Gloystein; Editing by Dale Hudson)