"In light of the continuing threats to the State of Qatar posed by foreign aggression, it is the policy of the United States to guarantee the security and territorial integrity of the State of Qatar against external attack," said an Executive Order signed by US President Donald Trump on Monday.

In the event of an attack on Qatar, the United States will "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 and to restore peace and stability," the order said.

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

The agreement 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.

Israeli Prime Minister Benjamin Netanyahu called Qatar's prime minister from the White House on Monday, apologizing for strikes and promising not to do so again, the United States said.

Netanyahu was in Washington to meet Trump, and had until then been defiant since ordering the September 9 strikes.

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.