BEIRUT — Islamic State militants re-entered the historic city of Palmyra in central Syria on Saturday for the first time since they were expelled by Syrian and Russian forces nine months ago.

The activist-run Palmyra Coordination network said the militants had nearly encircled the city.

The dramatic reversal in Palmyra comes days after militants in the Iraqi city of Mosul launched a major counterattack that surprised Iraqi soldiers, killing at least 20 and halting their advance.

Iraqi special forces units have entered eastern Palmyra, but their advance has been greatly slowed by both a desire to limit civilian casualties and the militants’ resilience. On Saturday U.S. Secretary of Defense Ash Carter announced that an additional 200 U.S. soldiers would be dispatched to Syria.