* LME copper slips into red after mounting Ukraine tensions
* SRB buying to support copper market
* Nickel on track for 11th weekly increase in 12
(Adds details, quotes; previous SINGAPORE)
LONDON, April 25 (Reuters) - Copper and other base metals slipped on Friday on worries that the crisis in Ukraine could deepen, but rising seasonal demand in China cushioned the losses.
A war of words ratcheted up after both Russia and Ukraine deployed troops close to their frontier. Ukraine's premier accused Russia of wanting to start World War Three and Russia said Kiev would face justice for a "bloody crime" in eastern Ukraine.
"With the weekend ahead of us and God knows what might happen in Ukraine, it is clearly a market factor and an increasing concern," said Stephen Briggs, metals strategist at BNP Paribas in London.
The mounting violence in Ukraine was having a broad impact on financial markets, capping investor appetite for some risky assets from commodities to equities.
Three-month copper on the London Metal Exchange fell 0.22 percent to $6,738 a tonne by 1016 GMT. It dropped into negative territory in early European trading after hitting a seven-week high of $6,780 in Asian activity.
Copper has risen 1.3 percent this week, its biggest weekly gain since the end of last month.
The escalating tensions in Ukraine are prompting some investors to lock in recent gains. But Helen Lau, senior metals analyst at UOB-Kay Hian Securities in Hong Kong, says copper could move higher in the near term, given the strength of seasonal demand in China.
"If you ask fabricators, their order books are all pretty good," Lau said, adding that prices may bounce back in the short term to $7,000, a level last seen in early March.
The most-traded July copper contract on the Shanghai Futures Exchange rose 2 percent to close at 47,920 yuan ($7,700) a tonne, after peaking at 48,020 yuan, also a seven-week high.
Purchases by China's state stockpiler, the State Reserves Bureau (SRB), had helped to boost premiums for copper held in Shanghai's bonded warehouses to $95-$115 a tonne over cash LME from as low as $65 in early April, Lau said.
The SRB has bought at least 200,000 tonnes of imported copper stored in bonded warehouses after global copper prices dived to multi-year lows in March.
The SRB move will provide continuing support to the copper market, Briggs said. "It does provide a floor. We know that were there to be large surpluses, it is likely that the SRB would absorb some of that; and when the price falls, the SRB is likely to be an opportunistic buyer."
LME nickel bucked the weaker trend, edging up 0.3 percent to $18,400 a tonne after fears of tighter supply lifted prices to $18,600 on Thursday, its highest in more than 14 months.
Nickel, the best-performing base metal this year, has gained more than 31 percent in 2014 as an Indonesian ban on exports of unprocessed nickel ore from January tightened supplies of the ore that top buyer China uses to make stainless steel.
For the week, nickel was up 2.6 percent, its 11th weekly gain out of 12.
Tin fell 0.2 percent to $23,669 a tonne after touching $23,849 on Thursday, its highest since early October. Tin has gained 1.1 percent for the week.
Three month LME copper
Most active ShFE copper
Three month LME aluminium
Most active ShFE aluminium
Three month LME zinc
Most active ShFE zinc
Three month LME lead
Most active ShFE lead
Three month LME nickel
Three month LME tin ($1 = 6.2489 Chinese yuan)
(Additional reporting by Manolo Serapio Jr in Singapore; Editing by David Goodman)