LONDON, June 24 (Reuters) - Asian spot liquefied natural gas (LNG) continued its upward trend this week as fears of further market tightening following a major outage at a Freeport plant and reduced Russian gas flows to Europe pushed buyers to cover short-term needs.

The average LNG price for August delivery into north-east Asia <LNG-AS> was estimated at $37 per million British thermal units (mmBtu), up $0.5 or 1.4% from the previous week, industry sources said.

"The market is back in a bullish mood, with prices up 60% in a couple of weeks, and we expect this to continue," said Alex Froley, LNG analyst at data intelligence firm ICIS.

Froley attributed the jump to lower Russian flows to Europe and the closure of a 15 million tonne per annum plant operated by Freeport LNG in the United States, as "neither of those gas sources look set to pick back up again in the near term".

Asian demand usually picks up at this time of year ahead of the summer cooling season and in preparation for restocking ahead of winter.

"We see stronger prices being mostly driven by the sudden tightness in global LNG supply - as opposed to demand - following the Freeport LNG outage and lower Russian flows, as this raises concerns of a slower rate of injection into EU gas storage," said Ryhana Rasidi, an analyst at data and analytics firm Kpler.

This would make some European buyers unwilling to give up supply to other importers, what will add more competition to the market and could keep Asian LNG prices elevated, she said.

Gazprom has reduced the capacity of the Nord Stream 1 pipeline that runs under the Baltic Sea from Russia to Germany by 60%, citing equipment problems. NS1 is expected to undergo maintenance in July.

S&P Global Commodity Insights assessed LNG prices for a delivered ex-ship (DES) basis into north-west Europe at $37.432 per mmBtu on June 23, at a discount of $3.50/mmBtu to August prices at the Dutch TTF hub, said Ciaran Roe, global director of LNG.

Roe added that discounts both for European LNG and the Japan Korea Marker widely used as an Asian benchmark have widened against TTF, with September JKM trading at a $5/mmbtu discount.

"The structures between TTF and international LNG prices are also in opposition to each other, with TTF being in contango while JKM is in a backwardation, as companies seek to short cover lost volumes from Freeport," he added.

Contango is where a futures price is higher than the spot price, while backwardation is the exact opposite, with the forward price of the futures contract lower than the spot price.

LNG freight spot rates continued to moved lower as vessel availability increased following the Freeport shutdown, with Atlantic rates estimated by Spark commodities at $55,500 per day and Pacific rates at $59,500 per day on Friday, down from $77,750 per day in the previous week. (Reporting by Marwa Rashad; Editing by Jan Harvey)