The US government has approved a request from Saudi Arabia to buy more than 19,000 bombs and smart bombs for its air force, the State Department said on Monday.

Congress will have to green light the deal, but the $1.29 billion dollar sale is likely to go through, and will replenish Saudi stores used up in their war on Yemen.

The Saudi-led operation against the Houthi militia in Yemen has proved controversial, amid frequent reports of civilian casualties on the ground.

But Washington has stood by its ally, which is also a player in the US-led coalition fighting the Islamic State group further north in Iraq and Syria.

The Pentagon's Defense Security Cooperation Agency said the Saudi air force's arsenal is low "due to the high operational tempo in multiple counter-terrorism operations."

The order includes 5,200 Paveway II laser-guided bombs in their GBU-10 and GBU-12 variants, along with 1,100 of the more modern, longer range GBU-24 Paveway III.

There are 12,000 general purpose bombs weighing between 500 and 2,000 pounds and 1,500 devastating 2,000-pound "bunker busters," the BLU-109 penetrator.

These are designed to smash hardened concrete structures.

In addition to the bombs themselves, the Saudis will receive thousands of "tail kits" to convert dumb munitions into satellite-guided smart bombs.

"The proposed sale augments Saudi Arabia's capability to meet current and future threats from potential adversaries during combat operations," the DSCA said, "providing these defence articles supports Saudi Arabian defence missions and promotes stability in the region."