The American Secretary of State Hillary Clinton has held talks with Syrian dissidents based in the US for the first time. The activists say they urged Ms. Clinton to call on the Syrian President Bashar al-Assad to step down immediately.The meeting came as diplomatic pressure intensified on the Syrian Government to hold its operations in the city of Hama in which about 140 people are reported to have been killed.Reports from Hama itself say a confrontation is continuing between Syrian Government forces and protesters with activists reporting several more people killed by tank and sniper fire. Government troops are said to be ringing the perimeter of in the city.