US President Donald Trump has signed an order vowing to defend key ally Qatar against attacks, the White House said Wednesday, in an extraordinary move following Israeli strikes on the Gulf state last month.

The Israeli strike angered Washington and spurred Trump to push Israeli Prime Minister Benjamin Netanyahu into accepting a Gaza peace deal this week -- and apologizing personally to Qatar.

"The United States shall regard any armed attack on the territory, sovereignty or critical infrastructure of the state of Qatar as a threat to the peace and security of the United States," said Trump's executive order.

"In the event of such an attack, the United States shall take all lawful and appropriate measures -- including diplomatic, economic, and, if necessary, military -- to defend the interests of the United States and of the state of Qatar."

The order was signed on Monday, the same day that Trump unveiled his 20-point peace plan for Gaza, but first appeared on the White House website on Wednesday.

Qatar's foreign ministry hailed Trump's move.

It said in a statement that the country "welcomes the signing of the US president's executive order recognizing attacks on its territory as a threat to American peace and security."

The agreement is unusual in that it is effectively pledging to protect one US ally against another.

It comes after an Israeli strike on the key US regional ally on September 9, targeting officials from the Palestinian armed group Hamas who were discussing a US peace proposal for the war in Gaza.

Qatar is one of the main mediators between Israel and Hamas.

- 'Deep regret' -

Netanyahu called Qatar's prime minister from the Oval Office of the White House on Monday, expressing "deep regret" for the strikes and promising not to do so again, the United States said.

The Israeli premier was in Washington to meet Trump, and had until then been defiant since ordering the Qatar attack.

The Axios news outlet said Trump had been angered by the attack and, seeing that Arab states had united around Qatar, used it to push Israel into accepting a Gaza deal despite Netanyahu's reservations.

Qatar is a key US ally in the Gulf and hosts the largest US military base in the region at Al-Udeid, which also includes a regional headquarters for elements of US Central Command.

Iran separately attacked the Al-Udeid airbase in June after US airstrikes on Tehran's nuclear progam.

Doha has carefully fostered a personal relationship with Trump since his return to power in January.

Trump visited the gas- and oil-rich state in May as he toured the Middle East on his first major foreign trip of his second term.

Qatar's royal family also gifted the United States a luxury Boeing 747-8 jet in May for Trump to use as Air Force One while he awaits the delivery of further presidential planes.

The deal raised ethical questions which the White House has rejected.

On Wednesday, White House Press Secretary dismissed a question about the role of Trump's son-in-law Jared Kushner in the Gaza plan and matters involving Qatar and other states that have invested heavily in his firm.

"I think it's frankly despicable that you're trying to suggest that it's inappropriate for Jared Kushner," Leavitt said.