KILIS, Turkey — Turkey has reached the end of its “capacity to absorb” refugees but will continue to take them in, the deputy premier said Sunday, as his country faced mounting pressure to open its border to tens of thousands of Syrians who have fled a government onslaught. The United Arab Emirates, meanwhile, joined Saudi Arabia in saying that it was open to the idea of sending ground troops to Syria to battle the Islamic State, raising the possibility of even greater foreign involvement in the five-year-old civil war. Turkish authorities say up to 35,000 Syrians have massed along the border, which remained closed for a third day Sunday.