US president Donald Trump is increasing pressure on Qatar to stop what he calls a "high level" of financial support of terrorism.
It comes as Secretary of State Rex Tillerson tries to calm the recent diplomatic crisis in the Persian Gulf.
Mr Trump's demand that there be "no more funding" by Qatar for extremist groups contradicted the message delivered on Friday by Mr Tillerson, who urged Qatar's neighbours to ease their blockade while calling for "calm and thoughtful dialogue".
Mr Trump, however, enthusiastically embraced the move by Saudi Arabia, Egypt and others to punish Qatar.
"The nation of Qatar, unfortunately, has historically been a funder of terrorism at a very high level," he said.
"We have to stop the funding of terrorism."
He echoed an allegation the Saudi-led group has used to justify cutting diplomatic ties to the gas-rich country.
Despite Mr Tillerson's plea for "no further escalation", Mr Trump's comments are likely to further embolden Saudi Arabia and others in their bid to isolate Qatar.
Mr Trump said that he, Mr Tillerson and military leaders decided during his trip to Saudi Arabia last month that a public rebuke was needed.
"The time had come to call on Qatar to end its funding - they have to end that funding - and its extremist ideology in terms of funding," Mr Trump said.
Mr Tillerson emphasised the economic, humanitarian and military damage he said the blockade was inflicting.
He said families were being separated, children removed from school and Qataris forced to deal with food shortages.
Mr Tillerson also said the blockade by Qatar's neighbours was "hindering US military action in the region and the campaign against ISIS".
But at the Pentagon, Navy Captain Jeff Davis said only long-term military planning was affected, and that operations at al Udeid air base had not been interrupted.
The Pentagon has been developing contingency plans in case there is any interruption, however, defence officials said.
Aircraft that fly out of Qatar - including fighter jets, drones and refueling planes - can be relocated, they said.
The escalating crisis in the Persian Gulf erupted this week when Saudi Arabia, the UAE, Egypt and Bahrain cut ties to Qatar, accusing the country of tolerating or even encouraging support for extremist groups, including al Qaida's Syria branch.
"Qatar has a history of supporting groups that span the spectrum of political expression, from activism to violence," Mr Tillerson said.
He credited Qatar's emir with making progress in curbing financial support and expelling terrorists, but added: "He must do more, and he must do it more quickly."
Increasing the pressure on Friday, Qatar's neighbours put 12 organisations and 59 people on a terror sanctions list and described them as being associated with Qatar.
The country has called the allegations "baseless".