UPDATE 3-French unions call new strike in pension showdown

* Sarkozy sticks to his guns on rise in retirement age

* Unions call for new strike for Sept. 23

(Recasts with union action, adds Elysee official)

By John Irish

PARIS, Sept 8 (Reuters) - French trade unions called another strike over unpopular pension reform after President Nicolas Sarkozy said on Wednesday he would not retreat on plans to raise the retirement age.

Following massive street protests and strikes that disrupted air and rail traffic on Tuesday, the unions raised the stakes with a call for more strikes and demonstrations on Sept. 23.

Hours earlier, Sarkozy said he was asking his government to make some amendments to the pension reform bill but that the rise in the minimum pensionable age, from 60 to 62, would stay.

"There is no question of going back on this," Sarkozy said. "Working a little longer is the most reasonable path."

Unions had warned of an escalation after Tuesday's protests, tapping into mounting unease over austerity measures in Europe as governments seek to reduce the large debts run up during the recession of 2008-09.

Officials at Sarkozy's Elysee Palace offices said that he had expected such a response and would not back down.

Sarkozy asked ministers to tweak the pension reform bill now before parliament, which also raises to 67 from 65 the age at which people are entitled to retire on a full pension, another key part of the plan to balance the system's finances by 2018.

SHOWDOWN

However, rather than affecting the bill's chief elements, the tweaks would accommodate complaints about the impact on people in physically gruelling jobs, or who started working very early in life, or who rely on many different pension schemes.

With the government aiming to pass the reform before the end of October, the unions have little time to manoeuvre.

"The president's announcement does not change the unjust and unacceptable nature of the proposed reform," a statement issued by the main unions said.

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For a TAKE-A-LOOK on French protests click [nECONOMYFR]

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The tone of Sarkozy's statement amd comments from Elysee officials suggested he was determined to face down the unions on what many consider the hallmark reform of his five-year presidency. He faces elections in early 2012.

The unions say Tuesday's protests in well over 100 cities on Tuesday drew a turnout of 2.5-2.7 million people while the official count is 1.12 million.

In any case the turnout was significantly bigger than for a previous protests.

The protests and strikes halved national rail services, disrupted underground train services in Paris and led to cancellation of a quarter of flights at Paris airports.

Without changes, the pay-as-you-go pension system would run up annual deficits of 100 billion euros ($128 billion) by 2050, the government says.

Opinion polls show two-thirds of voters think Sarkozy's plan is unfair, but two-thirds think strikes will make no difference.

"There is no question of letting anybody distort the reform, as it would imperil the rebalancing of our pensions," Sarkozy said. (Additional reporting by Marine Pennetier and Yann Le Guernigou; editing by Brian Love)