At least 12 Houthi rebels were killed when Saudi-led coalition airstrikes hit two pick-up vehicles in Rahabah district in Yemen's central province of Marib, a Ymilitary source at the scene said.The coalition airstrikes came as the Yemeni troops were advancing from two directions towards Al-Sadara mountain, the last rebel bastion in the southwestern district, the source told Xinhua news agency on Sunday.Fighting between the Yemeni army and the Houthi rebels also raged on in the neighbouring districts of Jabal Murad and Sirwah.Houthi-run al-Masirah TV reported 15 coalition airstrikes against the Houthi positions in those districts in the last 24 hours, without providing more details.The Houthis began in February a major offensive on Marib in an attempt to seize control of the oil-rich province, the government's last northern stronghold.The UN has warned that the offensive on Marib, which hosts nearly 1 million internally displaced people, could lead to a major humanitarian catastrophe.Yemen's civil war flared up in late 2014 when the Iran-backed Houthi group seized control of much of the country's north and forced the internationally recognised government of President Abd-Rabbu Mansour Hadi out of the capital Sanaa.The Saudi-led Arab coalition intervened in the Yemeni conflict in March 2015 to support Hadi's government.