A goodwill gesture to ease the plight of those hardest hit in Greece by tax increases and budget cuts has backfired spectacularly on the prime minister, Alexis Tsipras, with the country’s international creditors making clear he has acted out of step.

In the starkest case yet of how closely watched loan-reliant Athens is, lenders reacted with unusual alacrity on Friday after the leftist leader announced a one-off Christmas bonus for 1.6 million low-income pensioners.

“The programme includes clear commitments to discuss all measures related to programme objectives with the institutions in advance,” an EU spokeswoman said. “The commission was not made aware of all the details of the announcements before they were made. We will now need to study them.”

Retirees have been among those most affected by the gruelling regime of austerity the debt-stricken country has been forced to enact in exchange for over €300bn in emergency rescue funding.

Once unassailable, Tsipras’s own popularity has plummeted amid scenes of pensioners being teargassed and beaten as they took to the streets in protest. Under the scheme – announced in a televised address following a nationwide strike when anti-austerity demonstrations had swept the country – Tsipras said handouts of €617m (£518m) would be given to those living on €800 or less a month.

The Syriza leader, who was first catapulted to power as a strident opponent of the neoliberal policies inflicted on Greece, has long promised to make good on a pledge that he would seek to alleviate the lot of those most afflicted by the measures.

State minister Alekos Flambouraris, the 42-year-old politician’s closest mentor, said creditors had not been forewarned as the money came out of the surplus Greece had managed to achieve through stringent belt-tightening.

In a move indicative of the tensions between Athens and its creditors, Bild, the mass-selling German daily, poured scorn on the handout, saying: “Mr Tsipras has violated the agreements of the bailout programme.”

In recent weeks Greek-German ties have become increasingly strained, with Berlin’s powerful finance minister, Wolfgang Schäuble, reminding Athens repeatedly that Grexit, or exit from the eurozone, would beckon if it did not stick to the rules, implement reforms and attain tough fiscal targets.

Germany’s insistence that thrice bailed-out Greece commits to a primary surplus of 3.5% for a 10-year period after its current bailout accord expires in 2018 has added to the pressure as it has become ever clearer that Athens will have to adopt more austerity.

The demand has become a major sticking point in the continuing row over the International Monetary Fund joining the latest rescue programme – participation Berlin deems crucial if it is to continue disbursing funds. The recession-hit country has lost over 25% of its GDP – the biggest downturn to be experienced by an advanced western economy in peacetime – since its financial collapse seven years ago.

But social tensions are also spiralling.

“Tsipras is worried and that is why he made this move,” Grigoris Kalomoiris, chief policy maker at the union of public sector employees Adedy, told the Guardian. “Come January there will be more cuts to salaries and pensions in very real terms. We are all being pushed to breaking point. This, believe me, is the calm before the storm.”

Ignoring creditor anger, Tsipras’s beleaguered administration dug in its heels late on Friday, saying the bonus did not threaten fiscal targets and would not be rescinded. “It is up to the Greek government to distribute expenditure in the way it sees most fit and socially correct, as long as agreed goals are reached,” the prime minister’s office said. “Greece is not a colony.”