Greece has three days to reassure Europe and the International Monetary Fund it can deliver on conditions attached to its international bailout in order to receive the next tranche of aid.
The lenders are unhappy with progress Greece has made towards reforming its public sector, a senior eurozone official involved in the negotiations said.
A second source said an inspection visit might be suspended.
Athens, which has about €2.2 billion of bonds to redeem in August, needs the talks to conclude successfully.
If they fail, the IMF might have to withdraw from the €240bn bailout to avoid violating its own rules, which require a borrower to be financed a year ahead.
That would heighten the risk that concerted efforts by policymakers over the past nine months to keep a lid on the eurozone crisis could unravel, at a time when tensions are rising in other countries on the region's periphery.
Portugal's Finance Minister Vitor Gaspar, the architect of its austerity drive under an EU/IMF bailout, resigned in a potential blow to his country's planned exit from the EU-IMF rescue programme.
Political tension has also increased in Italy, where Prime Minister Enrico Letta called a government meeting after a coalition partner threatened to withdraw.
Athens and its creditors resumed talks to unlock €8.1bn of rescue loans, after a two-week break during which the government almost collapsed over redundancies at state broadcaster ERT.
Eurozone finance ministers are scheduled to meet on 8 July and discuss the situation in Greece, which is in its sixth year of recession and has seen unemployment surge to record highs.
"It is a very difficult negotiation", a senior Greek official participating in the talks said. "We're moving fast to wrap up as many issues as possible, as soon as possible."
Representatives of the EU-IMF-ECB troika have been holding serial meetings with government ministers in Athens, struggling to agree on a host of outstanding issues.
Athens has missed a June deadline to place 12,500 state workers into a "mobility scheme", under which they are transferred or dismissed within a year.
A shortfall of more than €1bn has emerged at state-run health insurer EOPYY, meaning automatic spending cuts may have to be agreed to bring it back on an even keel.
Greece and the Troika are also at odds over an unpopular property tax and a sales tax for restaurants.
Unemployment has hit a record 27% and Greeks have lost about a third of their disposable income at an average as a result of bailout-imposed austerity policies.