LONDON (Reuters/AFP) - World powers will do as much as they can to turn up the heat on the Syrian government, British Foreign Secretary William Hague said on Monday, calling President Bashar al-Assad's crackdown on eight months of protests "appalling and unacceptable." Hague held talks with representatives of Syrian opposition groups on Monday, intensifying British contacts with them just days after he appointed a former ambassador to lead London's coordination with Assad's opponents. The talks follow the expiry at the weekend of an Arab League deadline for Assad to pull the military out of urban centres, free political prisoners and start a dialogue under the 22-member group's initiative to end the bloodshed in Syria. Assad said in an interview published on Sunday he would not bow to international pressure to stop the crackdown on the protests against his rule in which the United Nations says 3,500 people have been killed. Turkish Prime Minister Recep Tayyip Erdogan warned Syrian President Bashar al-Assad Monday that his days as leader were numbered and he cannot remain in power indefinitely through military force. After reports that two buses carrying Turkish pilgrims came under attack in Syria, Erdogan said his one-time ally's defiant refusal to end a bloody crackdown on protesters had increased the prospects of foreign intervention. On Monday activists said Syrian forces killed two youths when they stormed a neighbourhood in the city of Homs looking for a football star who has been leading protests against Assad. Russia accused Western nations of undermining the chances of a peaceful resolution in Syria. Early on Monday gunmen opened fire on a convoy of Turkish buses carrying pilgrims inside Syria, Turkish media reported. Reports of the incident were fragmentary and Turkish authorities said they were still trying to establish what had happened. Some of the travellers said the attackers appeared to be Syrian soldiers. Alongside the mainly peaceful street protests against Assad, army deserters have launched a series of attacks against forces loyal to the president. Within hours of Assad ignoring Saturday's Arab League deadline, residents said two rocket-propelled grenades hit a major ruling party building in Damascus, the first such reported attack by insurgents inside the capital. A statement by the Syrian Free Army, comprising army defectors and based in neighbouring Turkey, initially said it carried out the attack but later withdrew its claim. Syrian Foreign Minister Walid al-Moualem denied any attack had taken place. But a witness said security police blocked off the square where the building was located and reported seeing smoke rising from it and fire trucks in the area. It was the second reported hit on a high-profile target in a week, underscoring a growing challenge to Assad who blames "armed terrorists" for the unrest in which authorities say at least 1,100 soldiers and police have been killed. In an interview with the British Sunday Times newspaper, Assad said he had no choice but to pursue the crackdown on unrest because his foes were armed. "The conflict will continue and the pressure to subjugate Syria will continue. Syria will not bow down," he said. Syria's main opposition group, the Syrian National Council, said that in the event of Assad's overthrow, it envisaged a transitional period lasting up to 18 months to agree a new constitution and hold a parliamentary election. But some prominent Assad opponents said more work was needed on uniting the opposition to bring about his downfall. Hague said his meeting with the opposition did not mean Britain was about to offer them formal recognition, "partly because there are differing groups." "There isn't a single national council as there was in Libya ... and the international community has not yet reached that point," he said. At the talks he met representatives from the National Council and the National Coordination Body, which has been more explicit in its opposition to military intervention in Syria. "We discussed the situation in Syria and the possibility of international protection to ... stop the bloodshed and provide protection for civilians," SNC chairman Burhan Ghalioun said. He told reporters the council wanted the West to work with Turkey and Arab states so that Assad could be "given a strong signal that he should leave and abandon power."