As Saudi-led airstrikes continue to hit targets in the capital of Yemen, Riyadh is boosting its fighting force.

The King of Saudi Arabia has ordered the Saudi National Guard to join the air force and army in the campaign against Houthi rebels allied with Iran.

At least 40 people are reported to have died in airstrikes on Tuesday, most of them civilians, according to health workers.

Dozens more have been wounded.

In key towns, fighters loyal to the exiled president Abd Rabbu Mansour Hadi are locked in battle with Houthi forces and other supporters of ex-president Ali Abdullah Saleh.

As Iran hints that a ceasefire may be on the cards, the US boosted its presence in the region, sending an aircraft carrier and a guided missile cruiser to waters near Yemen.