* Army sends extra special forces to two provinces * Widening U.S.-backed army offensive against al Qaeda * Turmoil threatens neighbouring Saudi Arabia (Adds government report of Saudi militant killed) By Mohammed Ghobari SANAA, May 12 (Reuters) - An air strike killed five al Qaeda militants in Yemen on Monday, state media said, and army reinforcements were sent to two regions in an expanding offensive against Islamist insurgents posing a threat to oil-producing Saudi Arabia and the West. Yemen is waging a U.S.-backed campaign to crush Al Qaeda in the Arabian Peninsula, the global jihadi network's regional wing. AQAP has mounted dozens of attacks on government officials, security forces and foreigners in recent months.