FRANKFURT (REUTERS) - European Central Bank chief Mario Draghi pledged indefinite stimulus on Thursday (Sept 12) to revive an ailing euro zone economy, tying the hands of his successor for years to come and sparking an immediate conflict with US President Donald Trump.

As Draghi's eight-year mandate nears its close, the ECB cut rates deeper into negative territory and promised bond purchases with no end-date to push borrowing costs even lower, hoping to kick-start the bloc nearly a decade after its debt crisis.

The bigger-than-expected stimulus will increase pressure on the US Federal Reserve and Bank of Japan to ease policy next week to support a world economy increasingly characterised by low growth and protectionist threats to free trade.

"You remember me saying that all instruments were on the table, ready to be used. Well, today we did it," Draghi told a news conference.

Yet there were doubts as to whether the ECB measures - most of the few remaining tools in its monetary policy arsenal - would be enough to boost a euro zone recovery in the face of a US-China trade war and possible disruption from Brexit.

Draghi acknowledged that the ECB's already gloomy projections did not encompass the possibility of a hard Brexit or a further escalation of the global trade war.

Thursday's moves also infuriated Trump, who just this week called on the US Fed to adopt a negative-rate policy.

"They are trying, and succeeding, in depreciating the Euro against the VERY strong Dollar, hurting US exports... And the Fed sits, and sits, and sits. They get paid to borrow money, while we are paying interest!" Trump tweeted.

LAGARDE CONSTRAINED

A 10 basis point cut in the ECB's deposit rate to minus 0.5 per cent was fully expected but the revived bond purchases exceeded many expectations because they are set to run until "shortly before" the ECB raises interest rates.

Given that markets do not expect rates to rise for nearly a decade, such a formulation suggests that purchases could go on for years, possibly through most of Christine Lagarde's term leading the bank.

"Today's decisions have anchored and enshrined the Draghi legacy in future ECB decisions," ING economist Carsten Brzeski said.

"Whatever it takes has just been extended by as long as it takes," Brzeski said, referring to the 2012 speech in which Draghi promised to do "whatever it takes" to save the euro, a bold move credited with holding the crisis-hit bloc together.

While conservative ECB policymakers had spoken out against more bond purchases in recent weeks, the decision suggests some of them eventually agreed, giving Draghi a majority for what is probably his last major policy move.

Underlining the need for action, the ECB cut its growth projections for this year and next, predicting growth at just above 1 per cent, below what is considered its natural potential.

The ECB's decision triggered a rally in euro zone bonds that will cut the cost of borrowing across the 19 countries that use the euro. The single currency itself firmed a touch after wild price swings during Draghi's news conference.

EFFECTIVE?

A simple rate cut would have increased the cost to commercial banks of parking their more than €1 trillion (S$1.5 trillion) worth of excess reserves at the ECB, a dangerous move since banks transmit the bulk of its policy to the real economy.

To offset that burden, the ECB promised even cheaper long-term funding and said it would introduce a multi-tier deposit rate to shield them from part of the ECB's punitive charge. That could leave lenders about €2 billion a year better off than previously, according to some estimates.

Euro zone stocks were little changed on Thursday, however, highlighting investors' doubts about the effectiveness of ECB policy, which can only prop up domestic confidence, not deliver a US-China trade deal or seal a Brexit agreement.

Indeed, Draghi stepped up his rhetoric in calling for governments to spend their way out of a slowdown, singling out Germany, which is obsessed with running a balanced budget.

"Now it is high time for the fiscal policy to take charge,"Draghi said.

"There was unanimity, namely that fiscal policy should become the main instrument."

Draghi has called for years for governments to do more to stimulate growth.

With the ECB's balance sheet already bloated and rates at record lows, analysts also questioned the effectiveness of more stimulus and suggested it could even work against the ECB.

"The key risk is that rate cuts could even backfire. Deeply negative interest rates could push up saving rates - see the surge in German savings, for instance," Shweta Singh, a managing director at TS Lombard, said.

"Crucially, there may be much less scope this time for the euro to edge lower and thus boost inflation expectations."