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WASHINGTON — The White House said Thursday that President Donald Trump has not made any final decisions regarding a U.S. response to a deadly chemical attack in Syria after convening with national security advisers at the White House.

The president told reporters ahead of the meeting that he was reviewing all options amid growing international calls for action against Syrian President Bashar Assad's regime. Defense Secretary Jim Mattis was among the top U.S. officials attending the White House meeting with the president.

"No final decision has been made," White House press secretary Sarah Huckabee Sanders said after the meeting ended. "We are continuing to assess intelligence and are engaged in conversations with our partners and allies."

Sanders said Trump is scheduled to speak with France's President Emmanuel Macron and British Prime Minister Theresa May later Thursday.

Trump, who earlier this month expressed his interest in pulling U.S. troops out of Syria, said that the recent suspected chemical attack on the Syrian town of Douma is leaving the U.S. with little choice but to respond.

The U.S. now has blood and urine samples from last Saturday's deadly attack that have tested positive for chemical weapons, according to two U.S. officials familiar with the intelligence, NBC News reported Thursday. The samples suggested the presence of both chlorine gas and an unnamed nerve agent, the officials said.

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"It's too bad the world puts us in a situation like that," the president said. "We've done a great job with ISIS, absolutely decimated ISIS, now we have to make further decisions. They'll be made soon."

Syrian opposition activists and aid groups said over the weekend that dozens of people had died in the suspected chemical weapons attack on Douma, and aid groups are blocked off from accessing the afflicted area.

Trump warned on Wednesday that airstrikes against Syria were imminent after a Russian diplomat pledged that U.S. missiles would be shot down and their launch sites targeted.

"Russia vows to shoot down any and all missiles fired at Syria," Trump said in a Wednesday tweet. "Get ready Russia because they will be coming, nice and new and 'smart.'"

Trump on Thursday dismissed comments he made earlier this week that he would make a decision on on Syria within "24 to 48 hours."

"Never said when an attack on Syria would take place," Trump tweeted. "Could be very soon or not so soon at all! In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS. Where is our 'Thank you, America?'"

In the United Kingdom on Thursday, British Prime Minister May convened parliament to discuss the UK's response to this week's attacks. British ministers are expected to back May's call to join military action threatened by the United States and its allies.

Macron also said on Thursday that he has "proof" that the Syrian government attacked the town of Douma with chemical weapons and said he would decide "in due course" whether to respond with air strikes.

Two U.S. officials familiar with the intelligence told NBC News Thursday that the U.S. also now has proof, in the form of blood and urine samples, which suggest the presence of both chlorine gas and an unnamed nerve agent.

Meanwhile, Secretary of State nominee Mike Pompeo, at his confirmation hearing Thursday, described Syria as a "failed state" that "poses a mounting threat to human rights, national security and regional stability — and it deserves an increasingly severe response."

Democratic lawmakers are urging the Trump administration not to move forward with any military strikes on Syria until a new "Authorization for Use of Military Force" is passed by Congress.

House Minority Leader Nancy Pelosi, D-Calif., said that while Trump's actions were "proportionate" last year when he ordered a missile strike on a Syrian airfield, Congress should first grant him new authorization before any new action proceeds.