Turkish forces who launched multiple artillery rounds near a U.S. Special Operations outpost in northeastern Syria on Friday have known for months that Americans were there, according to four current and former U.S. officials, raising questions whether Turkey is trying to push American troops farther from the border. The incident occurred on a hilltop base overlooking the town of Kobane as Turkey continues an operation launched Tuesday against Syrian Kurds, some of whom the United States has partnered with for years in its campaign against the Islamic State. The incursion has focused on an area 60 miles to the west...