The two were reported missing on Wednesday from a routine resupply mission, NATO forces headquarters said in a statement. It did not specify their nationality or say which province they had been in.

"We continue exhaustive search and rescue operations to locate our missing service members," spokeswoman Navy Capt. Jane Campbell said in the statement. "We are doing everything we can to find them."

Their families have been notified, and no further information will be released while the rescue operation continues and the incident is being investigated, the statement said.

NATO forces operating in western Afghanistan are mainly from Italy, Spain, the United States and Lithuania.

Separately, NATO said two American service members were killed Thursday by a roadside bomb in southern Afghanistan. It gave no further details.

The deaths bring to three the number of U.S. service members killed in the Afghan war so far this month. October was the deadliest month of the eight-year war for U.S. forces with 59 deaths reported.