Crucial discussions about Greece's indebtness have been postponed, because of health problems affecting both the prime minister and the finance minister.
Inspectors from the EU, European Central Bank and IMF had been due to review Greece's progress in meeting bailout conditions on Monday.
A new date for the trip will be determined later in the week.
Prime Minister Antonis Samaras is recovering from eye surgery, according to a government spokesman.
He underwent routine surgery on Saturday for a damaged retina. The operation was said to have been a success.
Mr Samaras was sworn in as prime minister on Wednesday to lead a pro-austerity coalition of three parties.
New finance minister Vassilis Rapanos is in hospital after apparently fainting on Friday.
His condition was said on Saturday to be "stable and improving". He was originally due to be sworn in on Saturday.
The BBC reports both men are expected to be discharged from hospital on Monday, but Mr Samaras is likely to be confined to his home for a week.
Greece is under huge international pressure to fulfil its bailout terms.
The new government is seeking to dilute the requirements imposed on it by the EU and by the International Monetary Fund, including extending the deadline to reduce its budget deficit by two years, to 2016.
The Greek reform programme was expected to be a major item on the agenda of the two-day EU summit in Brussels which begins this Thursday.