Member states will endorse a communiqué backing the plan to position missile defence bases in the Czech Republic and Poland, US officials said.
Albania and Croatia were invited to join the 26-member alliance. But Georgia and Ukraine will not be put immediately on the path to membership, while Macedonia was told it must solve a dispute with Greece before joining.
The communiqué on missile defence, circulated by US officials, acknowledges ballistic missile proliferation as "an increasing threat to allied forces, territory and populations".
It says the US-led system will make a "substantial contribution to the protection of allies". It also calls for steps to be taken to deploy a parallel NATO system to defend any countries not covered by the US scheme
The agreement emerged as US and Czech officials announced they had reached a deal to site a to base a missile defence radar on Czech soil, while Poland and the US resumed talks in Warsaw on locating 10 interceptor missiles in Poland.
