WASHINGTON – President Donald Trump said Thursday that a possible U.S. attack on Syria could be "very soon or not so soon," seeking to inject a sense of uncertainty after suggesting a response was imminent to a suspected chemical attack by Bashar Assad's regime.

"Never said when an attack on Syria would take place," Trump said in an early morning tweet. "Could be very soon or not so soon at all! "

Trump's message came roughly 24 hours after he warned on Twitter that U.S. airstrikes "will be coming," despite a pledge from Russia to shoot down American missiles aimed at its ally Syria.

"Get ready Russia, because they will be coming, nice and new and 'smart!' " Trump wrote Wednesday, referring to missile strikes that have appeared likely since the deaths of more than 40 Syrian civilians in the rebel-held town of Douma, near Damascus, last weekend.

Trump drew flak from his tweets on Wednesday from critics who pointed out that he had regularly attacked then-president Obama for previewing U.S. military strategy, revelations that Trump argued gave the enemy an advantage.

As president, Trump has boasted that he does not disclose his plans ahead of time. In April 2017, as he contemplated a strike in Syria, Trump said, "One of the things I think you've noticed about me is: Militarily, I don't like to say where I'm going and what I'm doing."

U.S. officials stressed Wednesday that planning for airstrikes has been careful and orderly, and has involved diplomatic and intelligence agencies as well as the Pentagon. But the same officials also said Trump's direct threat was unexpected.

Earlier this week, Trump said his administration was working on a response to the suspected chemical attack, including military options. On Monday, he said a decision would come in 24 to 48 hours, a time frame that has now elapsed and been complicated by plans for an international inspection of the attack area.

In his Thursday morning tweet, Trump also complained that the United States has not received enough credit from helping defeat the Islamic State terrorist group in the region.,

"In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS," Trump wrote. "Where is our 'Thank you America?'"