Angela Merkel and Francois Hollande presented a united front towards Greece, telling Athens it should not expect leeway on its bailout agreement unless it sticks to tough reform targets, Reuters reported.

The German and French leaders met in Berlin to fine-tune their message to Prime Minister Antonis Samaras, who begins a charm offensive in Berlin and Paris this week in the hope of persuading Europe's big powers that Greece deserves patience.

Merkel stuck to her policy of deferring to a report due in September on Athens' progress by the "troika" of international lenders before discussing flexibility on the bailout terms, but said it was vital "that we all stay true to our commitments".

"But we will, and I will, encourage Greece to continue on its path to reform, which has demanded a lot of the Greek people," she told reporters before a dinner with Hollande set to be dominated by Greece.

"We want, I want, Greece to be in the euro zone, it's a desire we have expressed since the start of the crisis. It's up to the Greeks to make the effort that is essential for that goal to be met," said France's Socialist president, standing alongside Merkel.

German sources who attended the working dinner later told Reuters the two leaders had vowed to work "together and with resolve" to overcome the euro zone crisis and had also agreed that "credibility" was the key to rescuing Greece.

A source close to the French presidency said the two leaders had wanted to have a "straightforward" talk on a host of hot-button topics, from the euro zone to Syria.

Hollande plans to visit Spain on August 30 and Italy in early September, the source said. Both countries have seen their borrowing costs shoot up this summer amid market fears that the euro zone may start to unravel.