The three allies used ships, a submarine and warplanes to launch a barrage of 105 guided missiles towards three chemical weapons facilities in Syria, officials said, including a research center on the outskirts of Damascus.

The strikes "will significantly impact the Syrian regime's ability to develop, deploy and use chemical weapons in the future," said Lieutenant General Kenneth McKenzie, director of the US military's Joint Staff, though he noted a "residual" element remained.

The missiles struck their targets within a minute or two of each other, McKenzie said, impacting around 4:00 am Syrian time (0100 GMT).

The overnight operation was the culmination of a week of frenetic planning at the Pentagon, with officials weighing the risks of various targets as President Donald Trump sent out mixed messages on what he wanted to do.

"All the options looked at ways to balance minimizing collateral damage against maximum effect. These three targets seemed to hit the sweet spot and do that," McKenzie said.

He said there were no known civilian casualties, but noted Syria had fired about 40 unguided surface-to-air missiles, most of which didn't launch until after the allied strike was over. These missiles may have come down in populated areas, he said.

"When you shoot iron into the air without guidance, it's going to come down somewhere," McKenzie said.

The Russian military said that 103 cruise missiles were fired including Tomahawk missiles, but that Syrian air defense systems managed to intercept 71 cruise missiles.

McKenzie countered that "the Syrian response was remarkably ineffective in all domains."

According to US officials, the operation comprised three US destroyers, a French frigate and a US submarine. The vessels were located in the Red Sea, the Gulf and the eastern Mediterranean.

The US Air Force fired air-launched cruise missiles from B-1 bombers, and French and British planes also shot cruise missiles toward the targets.

The operation was "precise, overwhelming and effective," McKenzie said, adding it will set their chemical weapons program back "for years."

Pentagon spokeswoman Dana White reiterated that the United States is only in Syria to fight the Islamic State group and does not want to get drawn into Syria's civil war.

"We do not seek conflict in Syria, but we cannot allow such grievous violations of international law," she said, referring to the suspected chemical attack.

"We successfully hit every target," she said.

"The strikes were justified, legitimate and proportionate."