The warning came at a bilateral meeting with French President Nicolas Sarkozy held in Berlin. Merkel also said European finance ministers should map out plans on a financial transaction tax by March, despite Britain’s pledge to block any such levy across the European Union.
"We must see progress on the voluntary restructuring of Greek debt," Merkel told a joint news conference with Sarkozy.
Merkel: No rescue plan – no aid
"From our point of view, the second Greek aid package, including this restructuring, must be in place quickly. Otherwise it won't be possible to pay out the next tranche for Greece."
Merkel said she would talk about Greece with International Monetary Fund chief Christine Lagarde. They are to meet in Berlin on Tuesday.
Banks, insurers and investment funds have been negotiating with the Greek government for weeks on a bond swap scheme which aims to cut its debt-to-GDP ratio to 120% from roughly 160% currently.
Meanwhile, Merkel backed the idea proposed by Sarkozy for the eurozone to introduce its own transaction tax if all EU countries cannot agree.
The Group of 20 top global economies has abandoned attempts to agree a transaction tax in the face of opposition from the United States, China, Britain and others.
Britain isolated on Tobin tax
Britain says it will veto such a tax at the EU level unless it is adopted on a global scale, fearful of the impact in the City of London financial centre. The issue could split the European Union at a summit planned for 30 January.
Without Britain's support, an EU-wide tax is impossible. But this would not stop a smaller group of nations within the bloc advancing with their own tax.
However, even within the eurozone, there is disagreement and countries such as Ireland want the tax to apply either to all 27 states or be dropped. Dublin has carved out a niche in London's shadow as a major centre for funds administration in Europe, and promoting the industry is a top government priority after the Irish banking crash.
Sarkozy vowed last week that France would push ahead with the tax unilaterally if other Europeans dither, and told reporters in Berlin: "If we don't set the example, it will not be done."
"We have no doubt we are going to start a trend in the eurozone for everyone to adopt this tax, which is exactly what is needed," he said.
Swiss chief banker resigns abruptly
Meanwhile, during the course of the Merkel-Sarkozy meeting the chairman of the Swiss National Bank, Philipp Hildebrand, resigned with immediate effect.
The development followed revelations that his wife Kashya bought €400,000 worth of US dollars in August, three weeks before the central bank intervened to cap the Swiss franc.