President Donald Trump on Thursday defended his surprise decision to declare victory over Islamic State militants and completely withdraw U.S. troops from Syria, amid criticism from some Republicans and concern from allies and some U.S. military commanders.

Meanwhile, Russian President Vladimir President said he largely agreed with Trump's assessment that ISIS had been defeated, and called Trump's move "the right decision" – but added that he was skeptical whether the U.S. would fully withdraw from Syria.

Putin said Moscow had not noted any signs of a U.S. withdrawal, and that the United States had many times said it was leaving Afghanistan, but still retained a presence there.

In a series of early posts on Twitter, Trump said he was fulfilling a promise from his 2016 presidential campaign to leave the Middle Eastern nation. The United States was doing the work of other countries, including Russia and Iran, with little in return and it was "time for others to finally fight," he wrote.

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Trump announced on Wednesday that he would begin withdrawing the roughly 2,000 U.S. troops from war-torn Syria, although the White House declined to offer a timeline.

Some of Trump's fellow Republicans fiercely criticized the move, saying they were not briefed ahead of time and that the move strengthened the hand of Russia and Iran, which both support Syrian President Bashar al Assad.

U.S. officials, speaking on condition of anonymity, have told Reuters that U.S. commanders on the ground are also concerned about the impact of a quick withdrawal.

"Getting out of Syria was no surprise. Ive been campaigning on it for years, and six months ago, when I very publicly wanted to do it, I agreed to stay longer. Russia, Iran, Syria & others are the local enemy of ISIS. We were doing there (sic) work. Time to come home & rebuild. #MAGA," Trump wrote on Twitter.

"Does the USA want to be the Policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing? Do we want to be there forever? Time for others to finally fight," he added.