(Updates with army, tribal fighters death toll in Omran and attack in southern Yemen)
SANAA, July 5 (Reuters) - The Yemeni air force bombed Shi'ite Muslim fighters north of Sanaa on Saturday in fighting that caused "a large number of casualties", local officials said, after a truce reached last month between the insurgents and government forces collapsed.
The fighting in northern Yemen, which has taken on a sectarian tone, is further destabilising a country struggling to overcome many problems, including a secessionist movement in its restive south and the nationwide spread of al Qaeda insurgency.
Shi'ite Houthi fighters, officially known as Ansarullah, blamed army units linked to the rival Sunni Muslim Islah party for breaking the June 23 ceasefire on Friday when government troops advanced on an area in al-Jouf province.
A Yemeni government official said the army's advance on the town of al-Safra in the province northeast of Sanaa had been prompted by the failure of Houthi fighters to vacate positions in compliance with the ceasefire.
Tribal sources in al-Jouf province, which is partly controlled by the Houthi rebels, said at least 18 people - 10 Houthis, five tribesmen and three soldiers - had been killed in clashes on Friday.
The fighting later expanded to the adjacent Omran province, where the Yemeni air force flew sorties and bombed Houthi positions around the provincial capital early on Saturday.
Local officials said "a large number of casualties" had been killed in Saturday's violence, including at least eight tribal fighters and four soldiers. The Houthis gave no figures for casualties on their side.
Despite appearing to falter after it took effect, the ceasefire had largely held with few reports of violations.
U.S.-allied Yemen, an impoverished country of 25 million that shares a long border with the world's top oil exporter Saudi Arabia, has been in turmoil since 2011 when mass protests forced veteran President Ali Abdullah Saleh to step down.
At least 200 people have been killed this year in battles pitting the Houthis - named after the tribe of their leader - against the government and Sunni tribal allies.
Officials say the Houthis, who have fought short but devastating wars with government forces since 2004, are getting weapons from Iran.
The Houthis deny this, saying they seek autonomy and less U.S. interference in Yemen's affairs.
Washington and Gulf countries are worried that further instability in Yemen could allow Al Qaeda in the Arabian Peninsula (AQAP), the Islamist group's regional wing, to consolidate its position and launch attacks overseas.
In southeastern Yemen, state news agency Saba reported that one soldier had been killed and four wounded on Saturday in a "terrorist" attack on a security compound in the Hajar area of Hadramout province. The agency gave no further details.
(Reporting by Mohammed Ghobari and Mohammed Mukhashaf in Aden,; Writing by Sami Aboudi; Editing by Sophie Hares and Pravin Char)