US president Donald Trump is ramping up 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 worst diplomatic crisis in the Persian Gulf in years.
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".
Only an hour later, Mr Tillerson sat in the front row in the Rose Garden as Mr Trump 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 tiny gas-rich country.
Despite Mr Tillerson's plea for "no further escalation", Mr Trump's sharp comments are likely to further embolden Saudi Arabia and others in their bid to isolate Qatar.
The State Department had said the US learned only at the last minute about the Arab nations' plan to cut ties.
But 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.
In a day of mixed messages and chaotic diplomacy, 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.
"We believe these are unintended consequences, especially during this Holy Month of Ramadan, but they can be addressed immediately," he said.
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, N avy Captain Jeff Davis said only long-term military planning was affected.
He said the diplomatic rift has not interrupted or curtailed operations at al Udeid air base, a launching pad for US military efforts in Iraq, Syria and Afghanistan.
But the Pentagon has been developing contingency plans in case there is any interruption, 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's neighbours have also criticised it for allegedly supporting groups such as the Muslim Brotherhood and Hamas.
"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."
The spat has led to one of the worst Gulf crises in decades, with suspended flights and regional ports closed to Qatari ships.
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".