"The nation of Qatar, unfortunately, has been a funder of terrorism at a very high level", Trump said.
"I decided, along with secretary of state Rex Tillerson, our great generals and military people, the time had come to call on Qatar to end its funding", he declared.
On Monday, Saudi Arabia, the UAE, Egypt and Bahrain led a string of countries that cut ties with Qatar over what they say is the country's financing of extremist groups and its ties to Iran.
Saudi Arabia and its Persian Gulf allies have targeted dozens of organizations and individuals with new terror-related sanctions - escalating a diplomatic crisis in the Middle East, particularly with Qatar.
The small Gulf Arab state is a critical global supplier of gas and hosts the biggest US military base in the Middle East.
Since Saudi Arabia and other nations cut off diplomatic relations with Qatar on Monday, triggering the worst diplomatic crisis in the Gulf in years, the US Army has insisted that it would not affect US military operations in the region.
The U.S. expects the countries to "immediately take steps to de-escalate the situation and put forth a good faith effort to resolve the grievances they have with each other", Tillerson told reporters at the State Department.
Iran, a close ally of Qatar, blamed the developments on U.S. President Donald Trump who recently visited the Middle East.
The country's supreme leader, Ayatollah Ali Khamenei, said the attacks would add to the hatred that Iranians harbor toward the US and Saudi Arabia.
Referring to the conversations that he had with leaders of the Gulf countries, Tillerson said it is clear to him based that the elements of a solution are available. "We ask Qatar and other nations in the region to do more and do it faster".
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The three attackers were wearing fake suicide belts consisting of plastic water bottles wrapped in grey duct tape. Butt hired the van used in the attack from a B&Q store in Romford in the morning of the attack , it said.
Tillerson also weighed in on behalf of Qatar, encouraging dialogue and cooperation between the states in the region.
Launching an extraordinary allegation against a key United States military partner, Trump derided what he called Qatar's "extremist ideology in terms of funding" terrorist groups, an accusation Qatar has repeatedly and vehemently denied.
Trump said the demand for Qatar to change its ways was made with Tillerson, who was at the White House for the president's news conference.
Qatar, for its part, has strenuously denied accusations that it is a supporter of terrorism, describing the moves to diplomatically isolate it as "unjustified".
Russian Foreign Minister Sergey Lavrov will today hold talks with his Qatari counterpart Mohammed bin Abdulrahman Al Thani.
The statement said the list was "an indication of the duality of Qatar policies".
Qatar's ambassador to Washington said on Thursday his government trusted Trump's ability to resolve the dispute.
"No more funding", Trump said.
Riyadh, Cairo and their allies accuse Qatar, the world's richest country per capita, of supporting militant Islamist movements across the region.
More than a third of the world's liquefied natural gas trade comes from Qatar, making it the world's largest producer and exporter of LNG. He also mentions that Iran has pledged to help Qatar with food and three of Iran's ports will be designated for Qatar. "Qatar remains critical for coalition air operations in the fight against ISIS and around the region", Pentagon spokesman Capt. Jeff Davis said in a statement.
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