BRUSSELS (AP) - Europe is “on thin ice” in its battle against COVID-19, German Chancellor Angela Merkel said Thursday as EU leaders agreed that vaccinations should be sped up to fight the highly contagious delta variant.

A stronger response to the pandemic was a main topic of discussion among European Union heads of state and government leaders at a meeting in Brussels, where they also acknowledged that the bloc's borders should be reopened in a cautious way.

In what might have been her last government declaration to the German parliament, Merkel noted that the number of COVID-19 cases in the 27-nation bloc continues to decline, while vaccination rates climb.

“But even though there is reason to be hopeful, the pandemic isn't over, in particular in the world's poor countries,” she said. “But in Germany and Europe, we're also still moving on thin ice.”

“We need to remain vigilant,” Merkel added. “In particular, the newly arising variants, especially now the delta variant, are a warning for us to continue to be careful.”

Upon his arrival in Belgian capital, French President Emmanuel Macron also urged European countries to remain “vigilant” in order to properly tackle the delta variant, and to adopt a coordinated approach when reopening their borders to third countries.

In a report issued this week, the European Centre for Disease Prevention and Control projected that the delta variant would account for 90% of all coronavirus infections across the continent by the end of August. The Stockholm-based agency said people who had only received one shot of vaccine were still vulnerable to the delta variant and that about 40% of people over 60 have yet to receive both vaccine doses.

The ECDC said its modelling scenarios estimate that any relaxation of the COVID-19 protocols currently in place “could lead to a fast and significant increase in daily cases in all age groups,” potentially reaching a similar peak to the surge last fall.

In their conclusions on COVID-19, EU leaders insisted on “the need to continue vaccination efforts and to be vigilant and coordinated with regard to developments, particularly the emergence and spread of variants.”

Germany has pressed for EU countries to form a joint position on quarantines for travelers from areas where named variants are particularly prevalent. This includes England, where the delta variant - first detected in India - already makes up a majority of cases.

EU leaders also praised the adoption of COVID-19 certificates ahead of the summer holiday season, with many calling for its coherent implementation across the bloc.

The EU has devised a joint digital travel certificate for people who are fully vaccinated, freshly tested, or recently recovered from the coronavirus. The free certificates allow people to move between European countries without having to quarantine or undergo extra coronavirus tests upon arrival.

Several EU countries are already using the system, including Belgium, Spain, Germany, Greece, Bulgaria, Croatia, the Czech Republic, Denmark and Poland. The rest are expected to start using it July 1.

Jordans reported from Berlin.