There is no need yet for ground troops in Yemen, the spokesman for a Saudi-led coalition which has conducted six days of air strikes against Shiite militia rebels said Tuesday.

"So far there is no need for land intervention," Brigadier General Ahmed Assiri told reporters, adding the need might arise "at any time".

Sunni-majority Saudi Arabia accuses Shiite-dominated Iran of backing the Houthis , who seized power in Yemen's capital Sanaa in February, forcing President Abedrabbo Mansour Hadi to flee to the port city of Aden.

He arrived in Riyadh last week after the rebels advanced on Aden, raising Saudi fears they would seize control of the entire country and take it into Iran's orbit.

But Assiri said air strikes north of Aden aim to block movement of the militia and their allies towards the southern city.

Similar operations in the country's north seek to "deny them to move toward the Saudi border," he added.

Naval vessels are blockading sea routes.

Aid agencies said on Tuesday they could not get assistance into the country on Saudi Arabia's southern frontier.

The closure of Yemen's international airports, and restrictions on seaports, are hampering delivery, Doctors Without Borders said.

Saudi Arabia welcomes all kinds of assistance for Yemen's needy, Assiri said, but it has to go through "diplomatic channels".

He said the movement of aid needs to be coordinated with the military "to make sure that we don't have any mistakes or any misunderstandings concerning the movement in the ports or airports or through the Saudi border."

Amnesty International said at least six civilians, including four children, had burned to death in air strikes on Tuesday morning in Ibb, central Yemen.

The London-based watchdog accused the coalition of "turning a blind eye" to civilian deaths.

Assiri reiterated accusations that the militia, who are "under pressure", have moved forces into villages but he said the coalition does not intend to kill civilians.

"Collateral damage can happen... but I confirm to you that the coalition takes all care", he said.