U.S. President Donald Trump's "chattiness" regarding the potential U.S. attack in Syria "ruined the element of surprise and allowed the regime and Russia to prepare" a for the attack, a source from the Syrian opposition forces told Haaretz on Wednesday.

To really understand Israel and the Middle East - subscribe to Haaretz

Trump warned Russia on Wednesday of imminent military action in Syria over a suspected poison gas attack, declaring that missiles "will be coming" and lambasting Moscow for standing by Syrian President Bashar Assad.

According to the Syrian opposition leader, Trump's tweets about the missiles "seems more like a show of force" which allowed Russia to prepare. Thus, if there's an attack, "it will not change the balance of power and will not help the Syrian people."

>> This is not a drill: Syria showdown could spark Israeli-Iranian and U.S.-Russian clashes ■ Revenge by Iran could push Israel to terminate its Syrian presence

The Syrian military has repositioned some air assets to avoid the fallout from potential missile strikes, U.S. officials confirmed to Reuters on Wednesday.

Open gallery view FILE - This Friday, April 7, 2017 file photo provided by the U.S. Navy, the guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea. With the Midd Credit: Ford Williams/AP

Trump tweeted in reaction to a warning from Russia that any U.S. missiles fired at Syria over the deadly assault on a rebel enclave near Damascus would be shot down and the launch sites targeted.

His comments raised the prospect of direct conflict over Syria for the first time between the two world powers backing opposing sides in the seven-year-old civil war, which has aggravated instability across the Middle East.

>> Israel on high alert, prepares for possible Iranian retaliation after strike on Syrian base ■ Revealed: The Iranian compound targeted by the 'Israeli strike' in Syria

"Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and 'smart!'," Trump wrote on Twitter.

"You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!" Trump tweeted, referring to Moscow's alliance with Assad.

In response, Russia's Foreign Ministry said: "Smart missiles should fly towards terrorists, not towards the lawful government".

Foreign Ministry spokeswoman Maria Zakharova said any U.S. missile salvo could be an attempt to destroy evidence of the reported gas attack in the town of Douma. Damascus and Moscow have denied any responsibility and say the incident is bogus.

Dozens of people in Douma died and hundreds were injured in the attack, according to the World Health Organization.

U.S. Defense Secretary Jim Mattis, striking a cautious tone after Trump's threat of missile strikes, said the United States was assessing intelligence about the suspected attack.

Asked if he had seen enough evidence to blame Assad, Mattis said: "We're still working on this."

The U.S. military was ready to provide military options, if appropriate, he added. It was unclear if his remarks reflected unease about Trump's apparent move toward military action.

Two U.S. government sources told Reuters the United States still did not have 100 percent solid evidence of what nerve agent was used in Syria and where it came from. However, there is some evidence it was sprayed from helicopters, they said.

In Moscow, the head of a Russian parliamentary defense committee, Vladimir Shamanov, said Russia was in direct contact with the U.S. Joint Chiefs of Staff about the situation.

The Syrian Observatory for Human Rights, a British-based war monitor, said pro-government forces were emptying main airports and military air bases. The Syrian military has also been repositioning some air assets to avoid the fallout from possible missile strikes, U.S. officials told Reuters.

The Russian military said it had observed movements of U.S. Navy forces in the Gulf. Any U.S. strike would probably involve the navy, given the risk to aircraft from Russian and Syrian air defenses. A U.S. guided-missile destroyer, the USS Donald Cook, is in the Mediterranean.

The Syrian foreign ministry accused the United States, which has supported some rebel groups in Syria's conflict, of using "fabrications and lies" as an excuse to hit its territory.

"We are not surprised by such a thoughtless escalation by a regime like the United States regime, which sponsored terrorism in Syria and still does," the state news agency SANA cited a ministry official as saying.

After the Douma attack, the insurgent group dug in there, Jaish al-Islam, finally agreed to withdraw. That sealed a huge victory for Assad, crushing a long rebellion in the eastern Ghouta region near the capital Damascus.

Britain ready

In London, British Prime Minister Theresa May said all the indications pointed to Syrian government responsibility for the attack and such "a shocking and barbaric act" could not go unchallenged.

"We're rapidly reaching an understanding of what happened on the ground," she told reporters. "We'll be working with our closest allies to consider how we can ensure that those responsible are held to account."

The BBC reported later that May was ready to give the go-ahead for Britain to take part in military action. She would not seek approval from parliament, the BBC said, despite calls from the opposition Labour Party for parliament to be given a say.

Oil prices jumped to their highest level in more than three years on Wednesday after Trump's warning, and U.S. stock index futures fell sharply due to alarm about a possible Russian-U.S. conflict over Syria.

White House officials did not immediately provide more detail about Trump's remarks. The U.S. Defense Department said it did not comment on potential military actions.