Columns of Turkish armor including tanks, armored personnel carriers, mobile artillery vehicles and engineering troops started crossing the Syrian border near the city of Jarablus around 11am on Wednesday morning. The move came after the start of Turkey's Operation Euphrates Shield aimed at liberating the city from ISIS. DEBKAfile's intelligence sources say that reports of US troops actively participating in the operation are not true. Meanwhile, Turkish artillery bombed positions of Kurdish militias to prevent them from taking over the city when the Islamic State fighters withdraw. Earlier in the day, US Vice President Joe Biden landed in Ankara. For details on Turkish President Tayyip Erdogan's diplomatic and military moves during the two weeks before the operation, read DEBKAfile's exclusive analysis.