After 18 consecutive months of buying government bonds to pump up the economy and raise inflation, the European Central Bank's holdings hit a landmark last week - €1 trillion. 

Whether the scheme - to which the ECB added €11.14 billion of government bonds in the week to September 2 - is working remains to be seen.

It is especially hard to gauge how bad the euro zone economy would have been without the purchases. 

One and half years after the ECB started buying state debt, results are mixed at best. Bank lending has rebounded but it remains sluggish and inflation is still stuck near zero. 

The ECB's policy-making body will meet again on Thursday.

While President Mario Draghi is not expected to make any announcement, he may hint at the prospect of extending the asset-buying scheme beyond its current end date in March. 

But prolonging the purchases again would be challenging when the ECB has already bought so much, because it risks further distorting market prices and even running out of eligible bonds. 

The ECB has already had to stop purchases in Estonia and found no bonds to buy in Luxembourg last month. 

In Portugal and Slovakia it has been acquiring fewer bonds than the rules of its programme dictate.

Germany, France, Italy and Spain were again over-represented as, overall, the volume of purchases slowed to €50.5 billion in August from €69.7 billion in July. 

To make room for more purchases, the ECB would consider raising a limit on how much of each bond issue it can own or looking at new asset classes, sources told Reuters earlier this year. 

Abandoning the so-called 'capital key' rule - which dictates that purchases be proportionate to how much each country has paid into ECB's capital - was seen as the least palatable option, the sources said. 

This is because it would leave the banks exposed to political and legal challenges.