WASHINGTON – Embattled Syrian President Bashar Assad and the rebels fighting for his overthrow are both participating in negotiations starting Wednesday in Geneva, despite an intensification of fighting on the ground.The UN-brokered talks, entering their second of three scheduled rounds, are meant to secure a political transition toward a national unity government for war-torn Syria, including a new constitution and fresh national elections. The sticking point in the talks is the fate of Assad himself, who has thus far shown no sign he is willing to leave office.Rebel groups, who have fought since 2011 for Assad’s overthrow, vow to continue the war without his departure. They, alongside their benefactors in the Arabian Gulf and in the West, consider Assad a war criminal and the primary agitator in the conflict.Under his rule, an estimated 300,000 people have been killed so far, and half of Syria’s pre-war population has been either internally displaced or sought refuge elsewhere.Tasked with negotiating on behalf of Syria’s diffuse rebel groups, the High Negotiating Committee (HNC) arrived in the Swiss city on Tuesday.Senior Assad regime officials were expected to arrive Thursday or Friday.Staffan de Mistura, the UN’s special envoy to the crisis, hopes to convince both parties to directly discuss the “mother of all issues”– a political transition– for the first time. Assad officials have thus far refused to negotiate his fate. But keeping the parties at the table will be a challenge in and of itself for de Mistura, as escalating conflict around the city of Aleppo– in violation of a two month-old cessation of hostilities between both sides– risks collapsing the effort.The US State Department said on Tuesday it was “very, very concerned” with reports that Assad’s forces, aided by Russia, Hezbollah and Iran, were positioning themselves to retake Aleppo, once Syria’s largest city. An HNC spokesman said on Tuesday that the new push on Aleppo was intended to dissuade rebel groups from attending the talks.On the other side, Assad’s foreign minister earlier in the week said that rebel cease-fire violations were intended to undermine the diplomatic effort. The al Nusra Front, a branch of al Qaida in Syria, operates in proximity to rebel groups in the Aleppo region.After hosting de Mistura in Tehran, Deputy Foreign Minister Hossein Amir-Abdollahian, blamed the “increasing activities of armed groups” for the violations. But Paris, which backs the opposition, blamed the other side.“It warns that the impact of the regime and its allies’ offensives around Aleppo and Eastern Ghouta are a threat to the cessation of hostilities,” government spokesman Romain Nadal said. The Eastern Ghouta is an opposition-held area near Damascus.US Secretary of State John Kerry has characterized these UN negotiations as the last, best chance for Assad to demonstrate he is “capable” of negotiating “in good faith.”He has also warned that the ceasefire will collapse, and that the war will escalate, should the talks fail.Reuters contributed to this report.