Trump offers to help resolve Gulf crisis, UAE tightens squeeze on Qatar

(Adds Trump phone call with Abu Dhabi's crown prince in paragraph 4)

* White House meeting possible, Trump says

* UAE sees possible embargo on Qatar

* Ratings agency downgrades Qatar's debt

* UAE official says more actions possible if needed

* Turkey OKs bill for supportive troop deployment to Qatar

* Qatar talks to Iran, Turkey about food, water supplies

By William Maclean and Tom Finn

DUBAI/DOHA, June 7 (Reuters) - U.S. President Donald Trump offered on Wednesday to help resolve a worsening diplomatic crisis between Qatar and other Arab powers as the United Arab Emirates invoked the possibility of an economic embargo on Doha over its alleged support of terrorism.

In his second intervention in the row in as many days, Trump urged action against terrorism in a call with Qatari Emir Sheikh Tamim bin Hamad al-Thani, a White House statement said.

"The President offered to help the parties resolve their differences, including through a meeting at the White House if necessary," it said.

Trump, in a later call with Abu Dhabi's crown prince, Sheikh Mohammed bin Zayed al-Nahayan, called for unity among Gulf Arabs "but never at the expense of eliminating funding for radical extremism or defeating terrorism," the White House said.

The UAE, Saudi Arabia, Egypt and Bahrain severed diplomatic ties with Qatar on Monday over long-standing allegations that Doha is courting Iran, which they deem the region's biggest external threat, and supporting Islamist groups, seen as the biggest internal danger.

The four countries also cut transport links to tiny gas-rich Qatar, disrupting food and other supplies and deepening uncertainty about the future of trade and investment ties.

In a sign of economic damage from the dispute, Standard & Poor's downgraded Qatar's debt on Wednesday as the country's riyal currency fell to an 11-year low amid signs that portfolio investment funds were flowing out because of the rift.

S&P cut its long-term rating of Qatar by one notch to AA- from AA and put the rating on CreditWatch with negative implications, meaning there was a significant chance of a further downgrade.

UAE Minister of State for Foreign Affairs Anwar Gargash told Reuters there would be more economic curbs on Qatar if necessary and said Doha needed to make ironclad commitments to change what critics say is a policy on funding Islamist militants.

He later told France 24 television that any further steps could take the form of "a sort of embargo on Qatar".

REGIONAL TENSIONS

Reiterating comments by Gulf officials, Gargash told Reuters there was no plan to seek a change in Qatar's leadership, only a change in its policies.