A senior US official said the United States has concluded that Russian Federation knew in advance of Syria's chemical weapons attack last week.
Instead, the U.S.is hoping to use the visit - the first by a Trump Cabinet official to Russia - to convey expectations to Moscow and allow Russians time to respond.
Putin said Moscow also wanted to see a "thorough" probe of the attack in the town of Khan Sheikhun conducted through the United Nations, and slammed U.S. accusations as reminiscent of the weapons of mass destruction claims leading up to the 2003 Iraq invasion.
But as USA jets fly in Syrian airspace, one big question is whether the United States and Russian Federation are keeping open a military communications channel to avoid an accidental clash.
Moscow says there is no proof that the Syrian military carried out the attack, and called the USA missile strike an act of aggression that violated worldwide law.
"The United States should revoke authorisations and re-impose sanctions on Iranian airlines found guilty of such support, and should bar USA companies from selling aircraft to Iran until the Iranian regime ceases using commercial airliners for illicit military purposes", the letter said.
Mattis warned that the future use of chemical weapons by Assad risked another US response - though he suggested this would only be for chemical weapons listed under an global agreement.
Another U.S. official cautioned that no final American determination has been made that Russian Federation knew ahead of time that chemical weapons would be used.
The Obama administration provided limited support for anti-Assad rebels, but never directly struck government targets after Obama called off such strikes four years ago, at a time when Trump also said attacking Assad would be a mistake.
"It is the right moment to talk about this, how the worldwide community, with Russia, Iran, Saudi Arabia, Europe, with the United States, can drive forward a peace process for Syria and avoid further military escalation of the conflict".
Mattis says Assad likely regrets chemical attack
Tillerson says it's unclear whether Russian Federation failed to take seriously its obligations in Syria or has been incompetent. Tillerson earlier met foreign ministers from the Group of Seven advanced economies and Middle Eastern allies in Italy.
US President Donald Trumps meeting with his Chinese counterpart Xi Jinping in Florida last week was "unbelievably helpful" and "productive", the White House said today.
Russia, a staunch ally of Syrian President Bashar al-Assad, has condemned U.S. actions as reckless and counterproductive.
Trump made a phone call to May and German Chancellor Angela Merkel and they discussed America's response to the suspected chemical attack by Syrian regime in which 87 people, including 31 children, were killed.
The top priority for the United States in Syria is to defeat the Islamic State group even before stabilizing the country, Secretary of State Rex Tillerson says. They are being taken by bus to rebel-held areas of northern Syria, accompanied by Russian forces.
"The reason for the strike was that alone", he said.
In appearances on the Sunday news shows, Tillerson said advances in North Korea's ballistic missile program concerned the USA the most.
Tillerson was asked if the USA would have any further intervention in Syria, he replied with, Washington's "first priority" is to defeat Islamic State.
Johnson put a positive spin on the outcome, saying there could still be sanctions on Russian military officers if an independent investigation into the chemical attack identifies perpetrators.
The Syrian opposition, which long accused the Obama administration of inaction, wants the U.S. attack to be the start of a more aggressive policy towards Assad.