He was speaking at a Berlin conference of foreign ministers of Southeast European countries which was also attended by Croatian First Deputy Prime Minister and Foreign and European Affairs Minister Vesna Pusic.
Westerwelle organised the conference together with Hungarian Foreign Minister Janos Martonyi and the Aspen Institute.
He said the fight against corruption and organised crime was crucial for drawing closer to EU membership, and that candidate countries should solve bilateral issues before accession.
If the two Germanies could unite, anything is possible, including peace in the Balkans, Westerwelle said, adding that he fully supported the efforts by EU High Representative for Foreign Affairs and Security Policy Catherine Ashton towards normalisation of relations between Serbia and Kosovo.
The key message is that your countries have a clear prospect of EU membership and we will support this. You belong to Europe and one day you will be EU members. This isn't just the German government's opinion, but parliament's as well. This is a clear message and I want you to understand it as such, he said.
Martonyi said Europe was indeed a successful continent on which there were many countries that wanted to join the EU. Germany and Hungary continue to support the Western Balkan countries. Their future is in the EU, he said.
Participating in the "Quo vadis Western Balkans?" panel, Pusic said the common perception of the Balkans was that of a region where there was a threat of conflict, adding that NATO and EU membership was the main mechanism of preventing conflict.
In itself, membership is the second most important goal, while the first is what membership and accession to those organisations brings in the context of stability and institution-building, Pusic said, highlighting the importance of a practical instead of a declarative political consensus.
Bosnian Foreign Minister Zlatko Lagumdzija said Euro-Atlantic integration was a key goal for his country, as it enabled a better organised state. We are irrevocably going in the right direction, sometimes not swiftly enough, but in the right direction nonetheless, he said.
US Deputy Assistant Secretary of State for European and Eurasian Affairs Philip Reeker said Europe was very important to the United States, as evidenced by Secretary of State Hillary Clinton's visit to Southeast Europe ahead of the US presidential election.
Also attending the Berlin conference were foreign ministers Enver Hoxaj of Kosovo, Nebojsa Kaludjerovic of Montenegro, Edmond Panariti of Albania and Nikola Poposki of Macedonia, Serbian Assistant Foreign Minister Zoran Vujic, and Ambassador Mehmet Gucuk of the Turkish Foreign Ministry.