The Pentagon is weighing a request from Turkish officials to help train and equip Sunni Arab fighters inside Syria as part of an effort to secure Turkey’s southern border, a long-sought goal of the Obama administration, according to U.S. officials.

The program would address part of the tense and long-running dialogue between Washington and Ankara over sealing the border, which Western governments have long complained is an avenue for extremists to leave Syria and travel internationally.

The Turkish request, made during a visit by the top U.S. military officer to Ankara last week, came just days before a blast Tuesday in the heart of a busy Istanbul tourist area killed at least 10 people and underscored the extremist threat in the region. Turkish officials blamed Islamic State for the blast, identifying the suicide bomber as a Syrian man in his 20s. […]