"It seems within the realm of possibility to me," she said after an EU summit in Brussels on Friday.
Shortly before her making her comment she had met with Polish Prime Minister Donald Tusk. The two leaders agreed on a follow-up meeting in Warsaw in June. Washington wants Poland to host the interceptor rockets for the US missile defence system, much to chagrin of Moscow.
As a counterweight to the planned establishment of an EU-Mediterranean union discussed at Friday's summit, Poland wants to include the possibility of Ukraine's accession to the EU. But Merkel said the idea was premature because Ukraine is not yet stable enough to join the EU.
The meeting in Brussels was the first bilateral meeting between Tusk and Merkel since their mid-December summit in Berlin when they agreed on closer cooperation.