The US state department has approved a deal for the support programme of Qatar's F-15 fighter aircraft at an estimated cost of $1.1bn.

In June, Qatar and the US signed a $12bn agreement in which Doha bought 72 of the multi-role fighter jets.

The Pentagon said it had informed Congress of the agreement involving design and construction work, including the development of cybersecurity, protection, support and other related services.

Congress now has 30 days to sign off on the deal.

In a statement, the US Defense Security Cooperation Agency said "Qatar is an important force for political stability and economic progress" in the Gulf region, and pointed out the strength of its relations with the Qatari Emiri Air Force (QEAF).

"A robust construction, cybersecurity, and force protection infrastructure is vital to ensuring the QEAF partners can utilize the F-15QA aircraft to its full potential," the statement said.

"Qatar will have no difficulty absorbing this support into its armed forces."

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The state department's approval came as the Gulf crisis between Qatar and four blockading countries - Saudi Arabia, Bahrain, the United Arab Emirates (UAE) and Egypt - continues.

Last month, US Secretary of State Rex Tillerson blamed the quartet for the impasse with Doha.

In an interview with Bloomberg, the top diplomat stressed that the leaders of the Saudi-led blockading alliance would have to take part in a dialogue to resolve the Gulf crisis.

"It's up to the leadership of the quartet when they want to engage with Qatar because Qatar has been very clear - they're ready to engage," he said.

The deal, which had been under negotiation since November 2016, was aimed at enabling Qatar's technical capacity and enhance security cooperation between Washington and Doha.