The White House warns Syria that further use of chemical weapons or barrel bombs could bring US military retaliation.

The United States has concluded that Russia knew ahead of time that Syria would launch a chemical weapons attack last week, a senior US official says.

President Vladimir Putin countered that Russia had information that the United States was planning to launch new missile strikes on Syria, and that there were plans to fake chemicals weapons attacks.

The US official offered circumstantial elements to back up his claim, but no concrete proof. And others in the Trump administration cautioned that final American determination had been made that Russia had advance knowledge of the attack, which killed more than 80 people and prompted retaliatory US cruise missile strikes.

AMMAR ABDULLAH/REUTERS A civil defence member breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria.

The senior official said on Monday (Tuesday NZ Time) that a drone operated by Russians was flying over a hospital as victims of the attack were rushing to get treatment. Hours after the drone left, a Russian-made fighter jet bombed the hospital in what American officials believe was an attempt to cover up the usage of chemical weapons.

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AMMAR ABDULLAH/REUTERS A man breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria.

The presence of the surveillance drone over the hospital couldn't have been a coincidence, the senior US official said, arguing that Russia must have known the chemical weapons attack was coming and that victims were seeking treatment.

The official, who wasn't authorised to speak publicly on intelligence matters and demanded anonymity, didn't give precise timing for when the drone was in the area. The official also didn't provide details for the military and intelligence information that form the basis of what the Pentagon now believes.

A senior White House official later said there is no US intelligence community consensus" yet on when Russia might have known of the attack. That official refused to be quoted by name, even though President Donald Trump has criticised media for citing anonymous sources.

The allegation of Russian foreknowledge is grave, even by the standards of the currently dismal US-Russian relations. And it is being made as Secretary of State Rex Tillerson travels to Moscow for talks on Syria and other matters.

Although Russia has steadfastly supported Syrian President Bashar Assad's government, and they've coordinated military attacks together, Washington has never previously accused Moscow of complicity in any attack that involved the gassing of innocent civilians, including children. The former Cold War foes even worked together in 2013 to remove and destroy more than 1300 tons of Syrian chemical weapons and agents.

Until Monday (Tuesday NZT), US officials had said they weren't sure whether Russia or Syria operated the drone. The official said the US is now convinced Russia controlled the drone. The official said it still isn't clear who was flying the jet that bombed the hospital, because the Syrians also fly Russian-made aircraft.

US officials previously have said Russians routinely work with Syrians at the Shayrat air base where the attack is supposed to have originated. US officials say the chemical weapons were stored there and that those elements add to the conclusion that Russia was complicit in the attack.

Last Thursday (Friday NZT), 59 Tomahawk missiles were fired on the government-controlled base in the United States' first direct military action against Assad's forces.

RUSSIA JABS BACK

Putin was speaking hours before US Secretary of State Rex Tillerson was due to arrive in Moscow for talks with Russia's foreign minister and days after denouncing last week's US missile strike on a Syrian air base as illegal.

Standing alongside Italian President Sergio Mattarella who was in Moscow for talks, Putin, when asked by a reporter if he expected more US missile strikes on Syria, said:

"We have information that a similar provocation is being prepared ... in other parts of Syria including in the southern Damascus suburbs where they are planning to again plant some substance and accuse the Syrian authorities of using (chemical weapons)."

He did not offer any proof for that assertion.

Russia has defended the Syrian government, a staunch ally, against US allegations it was behind the nerve gas attack in Syria's Idlib province last week which killed scores, saying there is no evidence to underpin such an allegation.

Putin said Russia would be urgently asking the global chemical weapons watchdog - the Organisation for the Prohibition of Chemical Weapons - to investigate the incident.

Speaking after Tillerson said earlier on Tuesday he hoped Russia would conclude it was wrong to align itself with the Syrian government, Putin said Moscow would tolerate Western criticism of its role in Syria but hoped that attitudes would eventually soften.

Putin's spokesman has said there are no scheduled plans for Putin to meet Tillerson on Wednesday (Thursday NZT), but Russian media have cited unnamed sources as saying that such a meeting will happen.

- AP, Reuters