KARKAMIS, Turkey, - Turkish tank units on Wednesday crossed the border and entered into Syria as part of a military operation backed by jets and US-led coalition warplanes to clear Islamic State from the Syrian border town of Jarablus, Turkish military sources said.A Reuters reporter at the border could see six Turkish tanks inside Syria and reported intense bombardments. Turkish jets and artillery have been carrying out shelling and airstrikes since Wednesday morning on Islamic State targets.Turkish-backed Syrian rebel forces have also entered Syria from Turkey as part of the operation to take back the town from Islamic State forces, a Syrian rebel commander told Reuters.