The US Secretary of Defense has said there is no doubt that the Syrian government was responsible for a poison gas attack last week on a rebel-held town, adding that it had used chemical weapons several times in the past.

During a press conference in Washington on Tuesday, James Mattis told reporters that independent authorities had documented and concluded the use of chemical weapons in an air strike on Khan Sheikhoun in Idlib province.

"There is no doubt the Syrian regime is responsible for the decision to attack and for the attack itself," Mattis said.

The US launched a barrage of cruise missiles against a government-controlled Syrian airfield as a response.

"We determined that a measured military response could best deter the regime from doing this again," Mattis said.

Mattis also said that the campaign against the Islamic State of Iraq and the Levant (ISIL) armed group is still the main focus of the US in Syria.

The Syrian government and its ally Russia have denied that Syrian aircraft had carried out the April 4 attack, which killed dozens of people, including many children.

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

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 the Syrian air base as illegal.

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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 been bombing rebel-held areas in Syria since September 2015.

Tillerson is the first senior US official to visit Moscow since Trump took office promising to seek closer cooperation with Russia, but as he arrived relations were already tense.

Earlier on Tuesday, Tillerson said he hoped Russia would abandon its support for Assad because actions such as the Khan Sheikhoun attack have stripped him of all legitimacy.

Tillerson made the remarks at the conclusion on Tuesday in Italy of a meeting of foreign ministers of the G7 and "like-minded" countries.

"It is clear to us the reign of the Assad family is coming to an end," he said shortly before leaving the Tuscan city of Lucca for Moscow.

"We hope that the Russian government concludes that they have aligned themselves with an unreliable partner in Bashar al-Assad."