STRASBOURG, France - Asserting his voice at NATO, President Barack Obama on Saturday welcomed Albania and Croatia to the alliance and declared to other nations that "the door to membership will remain open."

"It is a measure of our vitality that we are still welcoming new members," Obama said of NATO, which is marking its 60th anniversary at a summit dominated by the war in Afghanistan.

Obama, the one doing the welcoming, is himself new to the table. He is taking part in his first NATO summit and seeking support from allied nations toward the plodding effort in Afghanistan, where the new U.S. president is sending in more troops and civilian help.

One of NATO's stickiest political issues is how and where to grow. Germany, France and many other NATO nations fear any more NATO eastward expansion will further damage the alliance's ties to Russia.

Founded in 1949, NATO has added members since the collapse of the Warsaw Pact, its Soviet-dominated Cold War foe. In contrast to the alliance's previous eastward expansion, which infuriated Russia, Moscow has not objected to the inclusion of Albania and Croatia in NATO.

Albania and Croatia officially joined NATO this week. Obama praised them for having already deployed troops to the NATO-led force in Afghanistan, calling that commitment a sign that both countries will be strong contributors.