* Troops protest against air force head's sacking
* President Hadi trying to exert authority over armed forces
* State news agency hacked by southern secessionists
SANAA, April 7 (Reuters) - Members of Yemen's air force shut down the capital's airport on Saturday, stopping all flights in protest at the sacking of their commander, a half brother of former President Ali Abdullah Saleh, an aviation official said.
Military vehicles full of soldiers turned passengers away from Sanaa airport and prevented flights from taking off or landing, witnesses said. The action is a challenge to President Abd-Rabbu Mansour Hadi who replaced Saleh earlier this year.
On Friday Hadi sacked the air force head in a reshuffle intended to prise key military posts from Saleh's allies and restructure the armed forces, which split during the uprising against Saleh's rule, with some units openly siding with protesters.
Protests demanding the resignation of the air force commander, General Saleh al-Ahmar, earlier this year brought several airports to a standstill. Hadi on Friday shifted him to be assistant to the defence minister.
The airport's closure highlights the challenges Hadi faces in restructuring Yemen's army, upsetting the entrenched interests of Saleh's associates as well as those of a powerful general, Ali Mohsen al-Ahmar, some of whose allies were also sacked in Friday's shake-up.
General Ali Mohsen turned against Saleh early last year along with a chunk of the armed forces, sparking sporadic open combat on the streets of Sanaa with loyalist troops and tribal miltiamen that threatened to push the country into civil war.
A committee tasked with demilitarising Sanaa was on Saturday dismantling checkpoints set up by the warring factions in the western part of the city, to enforce a withdrawal of armed tribesmen and troops from the streets by the end of the week.
Previous such efforts have failed.
Hadi faces a sectarian rebellion in Yemen's north and an emboldened wing of al Qaeda concentrated in the south, which is also home to a separatist movement seeking to revive a socialist state Saleh united with the north in 1990.
Yemen's state news agency had been hacked on Saturday, apparently by southern secessionist sympathisers. Instead of the usual news feed, there were pictures of southern leaders and the former state's flag.
"Your turn has come all major Yemeni websites. If we do not see the southern flag waving above Yemeni sites we will eventually destroy them," read a statement posted on the site.
Some southerners accuse northerners of usurping their resources and discriminating against them.
They want no part in the united Yemen envisaged by neighbouring Saudi Arabia and the United States who threw their weight behind the power transfer plan under which Saleh left office after months of anti-government demonstrations that paralysed the impoverished state. (Reporting by Mohammed Ghobari; Writing by Isabel Coles; Editing by Robin Pomeroy)