"Today the news is awful," Mogherini said in an interview with media organisations in Brussels on the sidelines of a EU-UN conference that was meant to focus on the post-conflict situation in Syria.
"But this is a dramatic reminder of the fact that the situation on the ground still continues to be dramatic in many different parts of Syria," Mogherini said.
"And obviously, there is a primary responsibility there from the regime and first and foremost because it has the primary responsibility of protecting its people and not attacking its people."
"The issue of accountability is key for the EU but also for the conference itself."
Britain meanwhile condemned the attack in the northwestern town of Idlib and called for those responsible to be held to account.
"Horrific reports of chemical weapons attack in Idlib, Syria. Incident must be investigated and perpetrators held to account," Foreign Secretary Boris Johnson said on Twitter.
A Syrian security source on Tuesday said the gas attack report was a 'false accusation'.
The attack was the latest apparently involving chemical weapons since protests against Assad morphed into a bloody civil war in 2011, which has since left more than 320,000 people dead.
Damascus and the rebels blame each other for the previous chemical attacks, while Washington, London and Paris say Assad is responsible. Moscow, which backs Assad, says there is no clear evidence to back that up.
In 2013 the United States and Russia reached a deal to eliminate Syria's chemical weapons to avoid US air strikes against the Assad regime after hundreds of people were killed in a sarin gas attack that the US said killed 1,429 people.
The European Union and the United Nations are co-chairing a two-day conference to follow up on a London meeting last year which raised $11 billion (10 billion euros) for aid programmes to help the devastated country.
It is also looking at 'post-agreement assistance' once troubled UN-backed peace talks move forward, the EU said.
Former Italian foreign minister Mogherini admitted it was 'surreal' to be talking about the post-conflict situation but said the world had to plan ahead.
"I know it can sound surreal to start planning peace when in middle of conflict but... you hear this more than anything else, they want peace and want to get out of this terrible situation," she said.
The conference aimed to keep funding at the level of the London meeting for three years and check if London pledges have been delivered on.
"The message is on the human side we do not want to see the human support for Syria and Syrians diminishing," she said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)