Fierce clashes between rebels and pro-government forces killed dozens across south Yemen on Saturday, threatening to derail a humanitarian ceasefire drawn up to bring vital aid to the war-wracked country. The five-day truce initiated by a Saudi-led coalition that has bombarded the Iran-backed rebels for more than six weeks expires late Sunday, and Riyadh has already warned it was "ready to act" against any ceasefire violations. In the latest violence, at least 12 civilians were killed and 51 wounded when the Shiite Huthi rebels shelled several neighbourhoods in Yemen's third city Taez, military and local sources said. The clashes came after overnight fighting killed 26 rebels and militiamen loyal to former president Ali Abdullah Saleh as well as 14 pro-government forces, military sources said. The United Nations has expressed deep concern about the civilian death toll from the Saudi-led bombing as well as the humanitarian impact of an air and sea blockade imposed by the coalition. It says more than 1,500 people have died in the conflict since late March. Some aid has trickled into Yemen since the pause in fighting, but residents of areas where clashes persist complain they remain without the most basic supplies. The fighting in Taez overnight forced many to flee to the countryside. "Humanitarian aid hasn't reached Taez, where we haven't received fuel, food or medical equipment," said a government official in the city. However, UN refugee agency the UNHCR sent two planes loaded with aid to Sanaa on Saturday, airport sources said, while Red Cross and Doctors Without Borders (MSF) medical crews also flew in to the capital.