The disagreement could further delay the disbursement of the bailout funds which Athens badly needs to pay off IMF loans in June, bonds of the ECB maturing in July and increasing state arrears.
Last week, after months of negotiations, Greece and its lenders concluded a key bailout review, opening the way for debt relief that Greece has long desired.
But in a letter sent to the lenders last week, finance minister Euclid Tsakalotos said some of the additional demands could not be fulfilled, the sources said.
According to one of the sources, some of them were related to bad loans and to pension reforms.
Greek newspaper Ta Nea said the letter had been sent to EU Commissioner Pierre Moscovici, the ECB’s Benoit Coeure and the IMF’s Poul Thomsen.
In Brussels, an EU official confirmed that the two sides were still “working to finalise the measures” required after Athens raised its latest concerns.
The Ta Nea newspaper said specific areas of disagreement also included the privatisation of the country’s grid operator Admie and freezing the wages of essential services such as the coastguard and police.
Prime minister Alexis Tsipras has said a wage freeze “would have been a great injustice for men and women in the army, police, and coastguard who work 24/7 helping a country that is struggling with an unprecedented refugee crisis to stand on its feet”.
The measures have tested the left-led coalition, which has a narrow parliamentary majority with 153 of the 300 lawmakers. One lawmaker has resigned from Mr Tsipras’ Syriza party in protest over the measures.