"The horrific events of yesterday demonstrate unfortunately that war crimes are going on in Syria (and that) international humanitarian law is being violated frequently," Guterres said as he went into a conference on aid for Syria in Brussels.
Tuesday's attack on Khan Sheikhun in northern Syria left at least 72 people dead, including 20 children.
"There were also 17 women among the dead and the death toll could rise further because there are people missing," the Syrian Observatory for Human Rights said on Wednesday.
UK Foreign Secretary Boris Johnson went further in attributing blame for the attack, saying that "all the evidence" he had seen "suggests this was the Assad regime who did it in the full knowledge that they were using illegal weapons in a barbaric attack on their own people".
The attack left residents gasping for breath and convulsing in the streets and overcrowded hospitals.
Washington directly accused Bashar al-Assad of carrying out the "reprehensible" and "intolerable" attack, but also used the opportunity to take a swipe at the Obama administration's perceived inaction in Syria.
White House press secretary Sean Spicer described the atrocity as a "consequence of the past administration's weakness and irresolution".
Former US president Barack Obama famously drew a red line over Assad's chemical weapons use, only to backtrack and negotiate a deal with Russia to take Syria's chemical stockpile off the battlefield.
Despite the agreement, some crude chemical weapons remain in use.
|Over 70 people, including 20 children, were killed in Tuesday's suspected chemical attack [AFP]|
Spicer said an "extremely alarmed" US President Donald Trump was briefed extensively by security aides on the attack.
"Today's chemical attack in Syria against innocent people, including women and children, is reprehensible," Spicer said in a prepared statement.
Spicer added that the US was "confident" that Assad was to blame, however refrained from detailing how the Trump administration would respond.
"I'm not ready to talk about our next step, but we will get there soon," he said.
Spicer said that Assad remaining in power is a "political reality" and that there is no "fundamental option of regime change".
He did, however, suggest that a Syria without Assad in charge would be in the "best interest" of Syrians.
"The idea that someone would use chemical weapons on their own people, including women and children, is not something that any civilised nation should sit back and accept or tolerate," he said.
"I think it's in the best interest of the Syrian people to not have anybody who would do the kind of heinous acts.
"Any leader who treats their people with this kind of activity, death and destruction. Yeah. I don't think anyone would wish this upon anybody."
Syria and its chief ally Russia have denied responsibility for the attack.
The Syrian conflict began when the Baath regime, in power since 1963 and led by President Bashar al-Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.
According to independent monitors, hundreds of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.
The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.