Saudi warplanes have targeted separatist fighters in Yemen, killing seven in a counterattack against a sweeping advance by the United Arab Emirates-backed Southern Transitional Council.

The deaths on Friday are the first from coalition fire since the secessionist STC seized swathes of Hadramout and al-Mahra provinces last month.

Following the strikes, a military spokesperson for the STC said it was in a “decisive and existential” war with Saudi-backed Yemeni government forces, characterising the conflict as a fight against radical Islamism – a longtime preoccupation of the UAE.

Saudi Arabia and the UAE have for years supported rival factions in Yemen’s government-run territories, but the STC’s offensive has angered Riyadh and left the oil-rich Gulf powers at loggerheads.