LONDON — Britain has spent half a billion dollars over the last two years purchasing Paveway IV precision-guided bombs for use against Islamic State targets in Syria and Iraq, the British Ministry of Defence has revealed.

The MoD has not previously released the level of spending involved in maintaining stockpiles of the Royal Air Force's principle strike weapon against the Islamic State group. But, in response to a question from Defense News relating to Paveway IV production for the Saudi Arabian Air Force, the MoD revealed the extent of its purchases for the RAF.

"While unable to comment in detail about the supply of military equipment and services to Saudi Arabia, we can confirm that the £400 million (U.S. $500 million) in Paveway IV orders since 2015 does not include weapons for Saudi Arabia," an MoD spokesman said.

News of the Paveway IV orders come hot on the heels of a request by the British to the U.S. to purchase Lockheed Martin-built Hellfire missiles to replenish weapon stocks for Operation Shader, the name the U.K. gives its anti-ISIS mission.

All the munitions destined for operations are retrospectively paid for by the Treasury  on a usage basis.

The last Raytheon-built Paveway IV production order officially announced was in mid-2015 when then-Defence Procurement Minister Philip Dunne revealed that the MoD was buying nearly £50 million worth of weapons.

That deal is part of the approximately £400 million figure revealed by the MoD. Deliveries are expected to take place over the next few years.

Raytheon UK declined to comment on the Paveway IV orders.

It is unknown whether the orders take account of replacement weapons for the Paveway IV bombs diverted to Saudi Arabia's Air Force in 2015 from stocks originally earmarked for the RAF.

The MoD indicated Paveway IV and additional Brimstone missiles from MBDA had recently been ordered to ensure stockpile levels were maintained as intensive operations to oust ISIS from the Iraqi city Mosul and elsewhere continued.

"As operations to liberate western Mosul and Raqqa intensify, the RAF continues to provide precision close-air support to ground forces engaged in difficult, urban combat. As a result, the MoD has placed orders for Paveway IV and Brimstone 2 weapons to maintain our stockpiles," the MoD said.

Aside from the RAF, Saudi Arabia is the only customer for the high-precision, 500-pound Paveway IV bomb. Both nations deploy the weapon on their Typhoon and Tornado strike aircraft.

Paveway IV was co-developed by the British and U.S. arms of Raytheon for the RAF.

The bomb is assembled in the U.S. with a number of the components being supplied by Raytheon UK, Portsmouth Aviation, Thales UK and other British manufacturing companies.

Figures released to Parliament recently showed that strikes against ISIS using the precision-guided bomb far outweigh other weapons used by the RAF.

In the 12 months from Dec 2, 2015, the figures show 1,036 Paveway IV bombs were dropped, almost 10 times the number of dual-mode Brimstone missiles fired, and approaching four times the number of Hellfire strikes carried out by RAF Reaper remotely piloted vehicles.

Other versions of Paveway bomb, GBU-12 guided bombs, Storm Shadow cruise missiles and a small amount of 27mm cannon ammunition made up the rest of the weapons used in Operation Shader during the period.

The dual-mode Brimstone with seeker technology is being replaced by the latest variant of the missile known as the Brimstone 2. The new version of the weapon entered service on the Tornado last year and it is also being integrated on the Typhoon. The Brimstone 2 missiles are currently being built and stockpiled, with the dual-mode version being used in Iraq and Syria.

In a further move to maintain sufficient weapons stock, the MoD is looking to purchase 1,000 AGM-114 R1 and R2 Hellfire missiles, the U.S. Defense Security Cooperation Agency, or DSCA, said in a mid-March submission to Congress seeking approval for the $150 million deal.

The Hellfire missile is fired from Reaper drones employed on missions against ISIS. The missile also equips British Army Apache AH-64 helicopters, although the helos are not deployed in the conflict.

Interestingly, the DSCA submission said the weapons in the Foreign Military Sales deal would be drawn from U.S. government stocks rather than supplied by Hellfire contractor Lockheed Martin.

British officials played down suggestions the weapons were urgently needed but declined to discuss the state of the Hellfire stocks, saying it was an operational issue.

"The recent procurement of additional Hellfire missiles from the U.S. by the MoD was routine resupply," according to a British Embassy spokesperson in Washington.

That was followed up by the MoD spokesman in London saying that the "U.K. Reapers primarily undertake persistent intelligence, surveillance and reconnaissance tasks. All U.K. Reapers flying over Iraq and Syria are ready and able to deploy weapons as and when required."

A research note put out by U.S. financial service company Cowen in early April said the company found it "curious that the U.K. has requested 1,000 Hellfire missiles from U.S. stocks rather than directly from the manufacturer. We believe Hellfire production is at max capacity, so the British request is probably urgent and can only be met with a U.S. loan."

Whatever the reason for drawing Hellfire missiles from U.S. war stocks, the regular bulletins provided by the MoD detailing airstrikes against ISIS show a significant scaling back of Hellfire activity recently.

Two missiles were fired in the first 10 days of this month, but just one weapon in March, five in February and 28 in January — a figure which is close to the monthly average for last year.

Jen Judson in Washington contributed to this report.

Andrew Chuter is the United Kingdom correspondent for Defense News.

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