WASHINGTON, Jan 20 -- UN chief Ban Ki-moon expressed his grave concern on Tuesday about the deteriorating situation in Yemen, calling for an immediate end to all hostilities and the restoration of full authority to legitimate government institutions. In a statement issued through his spokesperson, he also condemned the recent kidnapping of Ahmad Awadh Bin Mubarak, Director of the Office of the President, and demanded the official's immediate release. The statement was issued amid reports that in the Yemeni capital, Sanaa, the presidential palace has been taken over amid renewed clashes and that the Prime Minister's residence was also under attack from the street. "The Secretary General is gravely concerned about the deteriorating situation in Yemen," said the statement, which called on "all sides to immediately cease all hostilities, exercise maximum restraint, and take the necessary steps to restore full authority to the legitimate Government institutions. "All sides must abide by their stated commitments to resolve differences through peaceful means to ensure the full implementation of the Peace and National Partnership Agreement in accordance with the outcome of the National Dialogue Conference and the GCC Initiative and Implementation Mechanism," according to the statement. "He also reminds all sides of their obligations under relevant Security Council resolutions," it said.