GENEVA (Reuters) - Fighting and air strikes have intensified in the Yemeni capital of Sanaa, where roads are blocked and tanks are deployed on many streets, trapping civilians and halting delivery of vital aid including fuel to supply clean water, the United Nations said on Monday. Some of the fiercest clashes are around the diplomatic area near the U.N. compound, while aid flights in and out of Sanaa airport have been suspended, the world body said in a statement following its appeal for a humanitarian pause on Tuesday.  The escalating situation threatens to push the barely functioning basic services ... to a standstill. These services have already been seriously compromised with the latest shock of the impact of the blockade,  it said, adding that fighting had also spread to other governorates, such as Hajjah.    