It still did not work in Eclipse with all solutions described above applied. The ant script was running well outside but in Eclipse I still got the error

"The prefix artifact for element artifact:dependencies is not bound..."

To resolve the issue I had to

1. Open "Window / Preferences" in Eclipse
2. Navigate to "Ant / Runtime"
3. Open the tab "Classpath" and click on "Ant Home Entries"
4. Click on the button "Add external jar" and choose the maven ant plugin jar I copied before into the lib folder of the ant eclipse plugin