Israel' military on Wednesday declared its newly acquired F-35 stealth fighters operational, making it the first country outside of the United States to put the ultra hi-tech jets into service.

The Israeli Air Force currently has nine jets out of the total of 50 it has agreed to buy.The aim of the purchase is to allow Israel to maintain its military superiority in the turbulent Middle East, particularly regarding its arch-foe Iran and the S-300 anti-aircraft missile system delivered to it by Russia.

Israel has also acknowledged carrying out air strikes in Syria since the outbreak of the civil war in the country six years ago to stop arms deliveries to Hezbollah forces.

Major General Amikam Norkin said Israel was "operating on a large scale on a number of fronts in a dynamic Middle East".

The F-35s added "another level" to the air force's capabilities, he said in remarks relayed by the army.

Made by US-based Lockheed Martin, the jets are the most expensive in history, with the first 33 sold to Israel at an average of about $110 million each.

The cost of the jets has been sharply criticised, including by US President Donald Trump, though Lockheed Martin says the price is gradually coming down.

Among its main features are advanced stealth capabilities to help pilots evade sophisticated missile systems.

The single-pilot jets can carry an array of weapons and travel at a supersonic speed of Mach 1.6, or around 1,200 miles per hour (1,900 kilometres per hour).