Afghanistan ? U.S. Marines fired smoke rounds Wednesday and armored vehicles maneuvered close to Taliban positions to test insurgent defenses ahead of an anticipated attack on the biggest militant-controlled town in southern Afghanistan.

A NATO spokesman in Brussels called on Taliban militants holding Marjah to surrender. But a Taliban spokesman boasted that the militants were prepared to “sacrifice their lives” to defend the town against the biggest NATO-Afghan offensive of the eight-year war.

The date for the main attack by thousands of Marines and Afghan soldiers has not been announced for security reasons. However, preparations have accelerated in recent days, and it appeared the assault would come soon.