US President Barack Obama said Friday that he had assured his Brazilian and Mexican counterparts that he understood their concerns over claims of US spying and has pledged to work with them to resolve the issue. \"What I assured President Rousseff and President Pena Nieto is that I take these allegations very seriously, I understand their concerns, the concerns of the Mexican and Brazilian people. We will work with their teams to resolve what is the source of tensions,\" he said, at the end of the G20 summit where he held talks with the two leaders on separate occasions.