BERLIN (Reuters) - The United States is moving toward permanently banning families from accompanying U.S. military and civilian personnel in Turkey, reflecting worsening security conditions there, two U.S. defense sources said on Wednesday.

The Obama administration in March ordered the families of U.S. military and diplomatic personnel to leave Incirlik air base, which has been used heavily in the fight against Islamic State militants, and other parts of southern Turkey. At the time, it said the move was not permanent.

The move affected about 670 dependents of U.S. military personnel in southern Turkey, while 100 others in Istanbul and Ankara were allowed to stay.

Now, military officials plan to designate deployments by all U.S. military and civilian personnel to Incirlik base in Adana and other sites in Turkey as "unaccompanied" tours, the sources told Reuters. The move was under consideration before Tuesday's suicide bomb attacks at Istanbul's main airport, which killed at least 41 people and wounded 239 others, the sources said.

"The change reflects the continued deterioration of security conditions throughout Turkey," said one of the sources, who was not authorized to speak publicly.

The change, which must still be finalized by the Defense Department, would mean that U.S. military deployments to Turkey would be reduced to one year from two, and troops would not be allowed to bring their families.

The U.S. military has about 2,200 service members and civilian employees in Turkey, about 1,500 of whom are posted to Incirlik base.

The change would not apply to U.S. personnel who are part of a "chief of mission" role or security cooperation team, the sources said.

The 100 dependents of U.S. personnel still in Turkey would be allowed to stay once the new rules took effect and would depart through natural attrition, said one of the sources.

The State Department on Tuesday warned U.S. citizens of increased threats from militant groups throughout Turkey and urged them to avoid traveling to the southeastern part of the country. It also extended the temporary departure orders for families of U.S. personnel working in Adana and Izmir province through July 26.

(Reporting by Andrea Shalal; Editing by Richard Balmforth and Leslie Adler)