"Stop funding (terrorism). Stop teaching hate. Stop the killing," Trump told reporters in the Rose Garden of the White House in a joint news conference with the visiting Romanian President Klaus Johannis.
In strong-worded opening remarks, Trump alleged that Qatar is funding terrorism at the "highest level" and asked other countries doing the same to stop with immediate effect.
"The nation of Qatar, unfortunately, has historically been a funder of terrorism at a very high level. And in the wake of that conference, nations came together and spoke to me about confronting Qatar over its behaviour," Trump told reporters at the news conference.
"So we had a decision to make: Do we take the easy road or do we finally take a hard but necessary action? We have to stop the funding of terrorism. 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 -- they have to end that funding -- and its extremist ideology in terms of funding," said the US President.
"I want to call on all of the nations to stop immediately supporting terrorism, stop teaching people to kill other people, stop filling their minds with hate and intolerance. I won't name other countries, but we are not done solving the problem. But we will solve that problem. Have no choice," Trump said.
Trump's comments came after his top diplomat Rex Tillerson appealed to Saudi Arabia and its regional allies to ease their blockade of Qatar, saying it is hindering US military actions in the region and the fight against the Islamic State.
In his remarks, Trump said he has just returned from a historic trip to Europe and the Middle East, where he worked to strengthen its alliances, forge new friendships and unite all civilised peoples in the fight against terrorism.
"No civilised nation can tolerate this violence or allow this wicked ideology to spread on its shores. I addressed a summit of more than 50 Arab and Muslim leaders, a unique meeting in the history of nations, where key players in the region agreed to stop supporting terrorism, whether it be financial, military or even moral support," Trump said.