By Jan Strupczewski and Jason Hovet

BRATISLAVA (Reuters) - Euro zone lenders pushed Greece on Friday to make up for lost time by speeding up a reform drive it agreed to in a bailout package before getting any new aid money.

Under a deal signed last year with euro zone countries, the European Central Bank and the International Monetary Fund, Greece can receive financial assistance of up to 86 billion euros by 2018 in return for agreed reforms.

With a deadline for important reforms this month and a closing window to available funds, euro zone officials sought to make sure Greece stayed on track to meet its obligations to receive 2.8 billion euros by the end of October.

"As the Eurogroup, we took the more general point and message that the summer is over, we really need to restart and pick up on the time lost," Jeroen Dijsselbloem, the head of the euro zone finance ministers, said after talks in Bratislava.

"And the Greek minister, our Greek colleague, was, I think, convinced there was a joint interest for all of us to keep this on track."

The Eurogroup, comprising euro zone finance ministers, had approved a tranche of 10.3 billion euros for Greece in May from the overall package. An initial 7.5 billion euros of that sum had been transferred to Athens with the rest available to Greece until the end of October if it meets some conditions.

With the highest debt to GDP ratio in the euro zone at more than 175 percent of national output, and after three international bailouts, Athens is hoping to show its lenders and investors the country is getting back on its feet.

But it still has work to do, including privatizations, energy market reforms and establishing an independent revenue agency that are part of 15 reforms it has promised.

"One could have wished to see more progress than we have… but what I state is that we have seen in recent days an intensification of efforts by the Greek authorities," EU commissioner for economic and tax affairs Pierre Moscovici said.

Other ministers meeting in Bratislava on Friday expressed concern that Greece was falling behind schedule.

Austrian Finance Minister Hans Joerg Schelling told reporters ahead of the meeting he was "not feeling very good about Greece". But German Finance Minister Wolfgang Schaeuble said Greece still had time to complete its work, saying Athens has in the past come through on required measures at the wire.

Athens is keen to keep the process on track as it also seeks debt relief and a return to debt markets following the inclusion of its debt in the ECB's quantitative easing program.

"There are serious concerns regarding the sustainability of Greek debt and it is in the interest not only of the Greeks but of the whole euro zone to find a lasting solution," ECB executive board member Benoit Coeure said.

(Additional reporting by Tatiana Jancarikova, Shadia Nasralla and Francesco Guarascio)