We are proud to have you as allies, you have deserved your place at the joint table, Obama said at the ceremony held before a session of the North Atlantic Council on the second day of NATO's two-day summit in Germany and France.
Also attending the ceremony were Croatian and Albanian Prime Ministers Ivo Sanader and Sali Berisha.
Croatia and Albania formally joined the alliance on 1 April when they deposited accession instruments in Washington. The US is the depositary of the Treaty of Washington.
We congratulate you on the progress you made in difficult reforms that have brought you to this point. Continuing on this path, you will get even closer to Euro-Atlantic integration, the US president said.
Obama said that Albania and Croatia had already done a lot for NATO, including 140 Albanian and 296 Croatian troops in Afghanistan.
Obama said this was not the last enlargement of the alliance, which now has 28 members, underlining that NATO's door would remain open to all countries meeting NATO standards.
I look forward to the day when we will be able to admit Macedonia to the alliance. The door to membership will remain open for other countries that meet NATO standards and can make a meaningful contribution to allied security, the US president said.
NATO Secretary-General Jaap de Hoop Scheffer also welcomed Croatia and Albania as new members.
You have worked on reforms for a long time, you are welcome to our ranks, you have deserved it, de Hoop Scheffer said.
The two countries were invited to join the alliance at its summit in Bucharest in April 2008. After that, two rounds of negotiations on their admission ensued, wrapping up on 7 May 2008.
It was agreed at the negotiations that Croatia would pay between three and four million euros into the alliance's joint budget, which is one of the lowest monetary contributions of member-states.
Upon the completion of those negotiations, Croatia sent a letter of intent to NATO with a defined time-table for the completion of necessary reforms, which was the final step before the signing of an accession protocol by NATO's 26 countries on 9 July 2008.
The signing paved the way for the ratification of accession protocols for Croatia and Albania in parliaments of the member states.
The sixth wave of the alliance's enlargement, that is the admission of Croatia and Albania, has brought the total number of members to 28.
Croatia defined its NATO membership as one of its foreign policy priorities in the mid- 1990s.
The first step on this road was taken in 1996 when Croatia applied for joining the Partnership for Peace Programme, which the alliance launched for aspirants in 1994.
Croatia was allowed to join PfP on 25 May 2000 in Florence, when Zagreb signed a framework document of PfP and Croatia was officially granted PfP member status.
On 15 May 2002, Croatia was invited to take part in the Membership Action Plan-MAP at an AECP (Euro-Atlantic Partnership Council) meeting in Reykjavik.
Within its preparations for NATO membership, Croatia adopted a long-term plan for the development of its armed forces from 2006 to 2015.
The plan envisages that the armed forces would include no more than 16,000 active military staff, 2,000 civilian staff, 2,000 voluntary conscripts per year, plus 6,000 in reserve.
As of 2010, only professionals will be used and the emphasis is on creating professional, mobile, deployable and financially viable forces that are interoperable with NATO forces.
Furthermore, eight percent of the land forces will be deployed and ready for deployment in international military operations, and 40 percent will be trained to participate in operations outside Croatia. This figure includes some 4,100 active military staff who will be equipped and trained for rotation in NATO, UN and EU missions.
Of that number, 700 troops will permanently participate in UN and NATO missions as of 2011.
In the long term, up to 1,600 troops will rotate in the NATO Response Force and the EU combat force. Croatia will train units that are ready to participate in NATO actions at any moment.
A reinforced motorized infantry battalion is expected to be organised and equipped by 2012.
By 2015, Croatia will outfit nine facilities for NATO requirements involving transit, exercises, assembly and emergencies. The facilities include barracks in Gasinci, Benkovac, Delnice and Vinkovci, airports in Pula, Zagreb and Zadar, and the naval port Lora in Split. These will not be NATO bases but will be used by the Croatian army.