The Turkish prime minister sharply criticized Syrian President Bashar al-Assad after an attack on buses carrying Turkish pilgrims Monday in central Syria wounded two people and brought a new low in an increasingly sour relationship between the two countries.

“You can remain in power with tanks and cannons only up to a certain point. The day will come when you’ll also leave,” Prime Minister Recep Tayyip Erdogan said at a conference in Istanbul. Responding to Assad’s repeated condemnation of other countries’ interference in Syrian affairs, Erdogan asked, “Why don’t you handle your own problem within yourself, without opening the way for outside interference?”

Turkey, which shares a border with Syria, was a close diplomatic and trade partner with the country before Assad’s government imposed a brutal crackdown against protests calling for greater political rights and freedoms.

Activists from the umbrella opposition group called the Local Coordination Committees reported that 18 people were killed by security forces across Syria on Monday, 14 of them in the central city of Homs, which has seen some of the worst violence and has a heavy presence of security forces. A United Nations estimate has numbered the deaths of protesters at more than 3,500 since demonstrations began in spring.

According to Turkish news television, the attack on the buses happened near Homs. The Local Coordination Committees showed video on their Facebook page of two buses with broken windows and Turkish paramedics treating injured passengers.

One of the drivers, Erhan Surmeli, told the Associated Press that a convoy of buses returning from the hajj, the Muslim pilgrimage to Mecca in Saudi Arabia, had stopped at a checkpoint. “Syrian soldiers emerged from behind sandbags and cursed Erdogan when we told them we were Turks. Then they suddenly opened fire at the bus,” he said.

It was not possible to verify the report independently, and armed opposition groups have also been increasingly active in recent weeks.

In London, British Foreign Minister William Hague condemned Assad, calling the violence in Syria “appalling and unacceptable.”

Speaking after a meeting with the Syrian National Council, a political opposition group that is seeking to topple Assad and form a transitional government, Hague said that Britain was not in a position to recognize the group formally because it was not yet representative of all aspects of the opposition movement. He called for greater unity among the Syrian political opposition and for protests to remain peaceful.