Four members of the UN Security Council -Britain, France, Germany and Portugal - will begin working on a Security Council sanctions resolution on Syria, Britain\'s deputy permanent representative to the UN has said. \"We believe that the time has come for the council to take further actions to step up the pressure against those who are responsible for the violence against the citizens of Syria,\" Philip Parham told journalists after a Security Council meeting on Syria which took place behind closed doors. \"So we will be working on a Security Council resolution which will include measures to apply that pressure to those that are responsible and we will be discussing that resolution with our colleagues on the council over the coming days,\" he said. U.S. deputy permanent representative Rosemary DiCarlo was quoted by media reports as saying that Washington would support the resolution. Earlier on Thursday, U.S. President Barack Obama called on embattled Syrian President Bashar al-Assad to step down, for the first time since the beginning of the unrest in Syria five months ago, and announced harsher sanctions against Syrian officials and companies. \"We have consistently said that President Assad must lead a democratic transition or get out of the way,\" Obama said in a statement. \"He has not led. For the sake of the Syrian people, the time has come for President Assad to step aside.\" French, German and British leaders, as well as the EU\'s top foreign-affairs official, Catherine Ashton, joined Obama on Thursday in calling on Assad to resign. In a phone conversation on Wednesday, Assad told UN Secretary General Ban ki-Moon that all operations against protesters in Syria had been halted. But as reports of new violence followed the pledge, Syria\'s authoritarian ruler faced mounting pressure from world powers on Thursday. In early August, the UN Security Council issued a statement condemning the violence in Syria, which is believe to have claimed some 2,000 lives, after failing to adopt a tough resolution due to objections from some members, including Russia and China.