France's U.N. Ambassador Francois Delattre said "with this attack, the Syrian regime has plumbed new depths of horror".
Tillerson's official itinerary in Moscow started with the meeting with Lavrov, in an ornate hall in a foreign ministry-owned residence.
Five days before the Khan Sheikhoun attack, Haley had indicated the U.S. had ditched the Obama administration's policy of removing Assad. "I like to think they didn't know".
One of Lavrov's deputies was even more undiplomatic.
Tillerson, however, used his opening remarks to portray a more diplomatic tone in which he called the talks "an important moment in the United States' relationship with Russian Federation".
"As a whole, the administration's stance with regards to Syria remains a mystery".
Tillerson kept to more calibrated remarks, saying his aim was "to further clarify areas of sharp difference so that we can better understand why these differences exist and what the prospects for narrowing those differences may be". So, I felt we had to do something about it.
The dim view of U.S.
Moscow's hostility to Trump administration figures is a sharp change from past year, when Putin hailed Trump as a strong figure and Russian state television was consistently full of effusive praise for him.
The White House pointedly said yesterday that it had seen credible reports that Moscow backed an election-day attack in Montenegro past year.
Trump responded to the gas attack by firing 59 cruise missiles at a Syrian air base on Friday.
The Britain-based Syrian Observatory for Human Rights says government troops and allied forces have already entered Madaya, one of the opposition-held towns evacuated of rebels.
Moscow has stood by Assad, saying the poison gas belonged to rebels, an explanation Washington dismisses as beyond credible.
Putin said afterwards that the strike had seriously hurt the Russo-American ties.
Trump came to the presidency promising to seek closer ties with Russian Federation and greater cooperation fighting against their common enemy in Syria, Islamic State.
Tillerson noted the low level of trust between the two countries.
The United States and its European allies imposed financial sanctions on Russian Federation in 2014 after Putin seized territory from neighboring Ukraine.
The presidents of the United States and Russian Federation on Wednesday both presented souring views of the relationship between their two countries, exchanging sharp words as Moscow extended an icy welcome to the United States' top diplomat in a face-off over Syria.
A senior Russian diplomat says the United States and Russia are again exchanging information on a hotline created to prevent midair incidents over Syria, but that the formal agreement on such exchanges has not been renewed.
Russian Foreign Minister Sergey Lavrov said President Trump's administration has found itself under pressure from those who "want to prevent it from curing the wounds inflicted by Barack Obama's administration and are using the Russian card in internal political struggle".
A senior Russian diplomat says Moscow believes that an global probe into last week's chemical attack in Syria should include experts from Brazil, India, Iran and other nations. "But when I see people using disgusting, awful chemical weapons ... and see these attractive kids that are dead in their father's arms, or you see kids gasping for life ... when you see that, I immediately called (Defense Secretary) General Mattis". But any such expectations have crashed into reality amid the nasty back-and-forth over Syria and ongoing US investigations into Russia's alleged interference in America's USA presidential election.
US Ambassador to the United Nations, Nikki Haley, called on Moscow to stop protecting Assad, adding the US wants to work with Russian Federation toward a political solution for Syria.
They plan to press Trump to explain whether he will conduct future attacks against Syrian President Bashar al-Assad's regime as well as push for Congress to reassert its constitutional authority to declare war, congressional aides and lawmakers on the call told CNN. He said working groups would be established to improve U.S.
The dim view of US-Russian ties from both Trump and Tillerson reflected the former Cold War foes' inability to forge better relations, as Trump until recently has advocated. The Federal Bureau of Investigation is investigating whether any Trump campaign figures colluded with Moscow, which the White House denies.