Syrian opposition forces launched an assault on a regime-controlled border crossing into Turkey on Friday. The main armed opposition group in Syria, the Free Syrian Army (FSA) fighters mounted an early morning attack to seize the town of Keseb and its Turkish border gate, opposition sources told an Anadolu Agency correspondent.Sound of clashes were heard in the Turkish border town of Yayladagi and villages along the border.Three regime-controlled border posts have reportedly been captured by the opposition.A Turkish commander, who wanted to remain anonymous, also confirmed to our correspondent that there were clashes taking place.Turkey has put its security forces on alert and warned its citizens living near the border.The war between the Free Syrian Army and forces loyal to the embattled president Bashar Assad has been in an uneasy stalemate since late 2013.Recently, regime-affiliated troops reportedly captured Yabroud, one of the last opposition strongholds in the mountainous Qalamoun region along the Lebanese border.On February, the National Coalition for Syrian Revolutionary and Opposition Forces made a reshuffle in FSA's general staff, replacing former chief Salim Idriss with veteran field commander Abdullah al-Bashir.The Syrian civil war, which entered its fourth year last week, has claimed more than 140,000 lives, according to the London-based Syrian Network for Human Rights (SNHR).Syria has been gripped by almost constant fighting since President Bashar al-Assad's regime responded to anti-government protests in March 2011 with a violent crackdown, sparking a conflict which has spiraled into a civil war.