Negotiations begin in earnest, with Obama and Kerry praising Israeli-Palestinian summit but warning of big challengesIsraeli and Palestinian negotiators are beginning intensive talks in Washington on Tuesday aimed at reviving the moribund Middle East peace process amid warnings that this could be the last chance to reach an agreement to end the historic conflict.As the two teams met on Monday evening for an iftar meal hosted by the US secretary of state, John Kerry, to mark the end of the Ramadan fast, the preliminary talks were welcomed in the first formal, albeit cautious, statement from President Barak Obama since the two sides agreed 10 days ago to sit down together."This is a promising step forward, though hard work and hard choices remain ahead," Obama said.