The two sides clashed early on Monday morning near the palace of President Abdrabbuh Mansour Hadi, with gunfire heard across the city, residents said. Houthi forces seized the president's chief of staff on Saturday. Yemen has been plagued by instability since mass protests forced former President, Ali Abdullah Saleh, to step down in 2011. Scores of people have been killed in clashes between the Shia Houthis and Sunni militants. UN deal President Hadi's chief of staff, Ahmed Awad bin Mubarak, and two of his guards were abducted in the centre of Sanaa. The Houthis said they acted to prevent a UN-brokered deal between them and the presidency being broken. A statement from the Houthis warned President Hadi of "a series of special measures" they were prepared to take to prevent the breaking of the deal. The two sides agreed a truce in September last year after mounting clashes in Sanaa. The group was formed in 2004 to win greater autonomy for their home region of Saada, and to protect followers of Zaidism, the branch of Islam they adhere to. Their critics say they are a proxy for Iran, a charge the rebels deny. BBC