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US Secretary of State Rex Tillerson has cancelled a meeting in Mexico for the Organisation of American States to concentrate on a diplomatic crisis gripping the Middle East's Gulf region.

Deputy Secretary of State John Sullivan will head the US state department at the regional meeting in Cancun due to be held from Monday to Wednesday.

"The secretary of state will continue his efforts to de-escalate tensions in the Middle East region through in-person meetings and phone conversations with Gulf and regional leaders," the State Department said in a statement.

Saudi Arabia, the UAE, Bahrain, Egypt and others severed diplomatic ties and enforced a blockade on Qatar almost two weeks ago, accusing Doha of supporting extremist groups.

Qatar - a key ally of Washington and host of the Middle East's largest US airbase - denies the accusations.

But last week, US President Donald Trump expressed support for the Saudi-led allegations, charging that Qatar had "historically been a funder of terrorism at a very high level".

Pentagon and state department officials have since scrambled to reassure the emirate - which hosts the the command headquarters for anti-Islamic State group operations in Iraq and Syria - that Qatar is still a key US ally.

On Thursday, the US agreed to a $12 billion warplane sale to Qatar, reaffirming its support.

Tillerson "has made more than a dozen phone calls and participated in several in-person meetings" in efforts to tamp down the crisis, the state department said.

"The secretary will continue these efforts."