This is a crucial moment for the future of your country, Rehn told reporters after meetings in Athens.
No member of the euro zone area can live permanently beyond its means ... Either you keep your debt under control or your debt starts controlling you.
Rehn said he sensed a strong determination by the Greek government to reform the country but said additional measures were needed as soon as possible if Greece was to solve the country's debt crisis.
Given that risks related to macroeconomic and market developments are materializing, additional measures of fiscal consolidation are necessary, he said.
Referring back to earlier promises by the European Union, Rehn said countries using the single European currency remained ready to provide coordinated support for Greece should the need arise but declined to give details.
The European Union has the ways and means to ensure financial stability in the euro area, he said, adding that the Greek government had not asked for financial help.
Finance Minister George Papaconstantinou said plans to deal with Greece's debt burden involved dangers and risks and some of these dangers are now visible but the government would not waver in its commitment to reducing the deficit.
The government will do whatever it takes, including (taking) new measures so that the target for reducing the budget deficit by 4 percent this year is observed unwaveringly, he said.
The two men discussed reforms to the pension system, the budget and the statistics service, he added. Labor Minister Andreas Loverdos told local radio that pensions may be frozen this year.
Prime Minister George Papandreou earlier called on citizens to bear the pain required to put the country back on track, saying corruption and impunity were at the heart of its problems.
The crisis in our country is not limited to our fiscal problem. It is only the tip of the iceberg, Papandreou said in a televised address to the cabinet.
Today we ask Greek men and women to enlist in our common cause to save our country and the overwhelming majority of our citizens are willing to do it despite the price and despite the burden.
(Additional reporting by Lefteris Papadimas; Writing by Paul Hoskins; editing by Myra MacDonald)