The fresh call came a day after the suspected chemical attack on the town of Khan Shaykhun in Syria's Idlib that killed over 70 people.
Britain and France on Wednesday renewed their call for Syrian regime leader Bashar al Assad to leave office.
The fresh call came a day after a suspected chemical attack – allegedly by the regime – killed scores of people in a rebel-held area, eclipsing an international conference to promote peace.
Foreign ministers Boris Johnson of Britain and Jean-Marc Ayrault of France spoke during the international conference on Syria, which the European Union convened in Brussels in a bid to shore up stalled peace talks between Assad and his rivals.
"I simply don't see how Bashar al Assad can remain in charge after what he has already done," Johnson said. Of the 400,000 people who are estimated to have been killed in Syria, he is responsible for the vast majority of the butcher's bill," Johnson said.
Ayrault said the attack was a test for the new US President Donald Trump and his stance on Assad.
The future of Assad, backed by Russia and Iran, has always been the main point of contention blocking progress in talks.
The war has raged for more than six years, displacing millions and throwing civilians into dire humanitarian conditions.
TRT World's Sarah Morice reports from Brussels.
UN calls for humanitarian aid
"The need for humanitarian aid and the protection of Syrian civilians has never been greater. The humanitarian appeal for a single crisis has never been higher," United Nations' Secretary General Antonio Guterres said.
The UN has called for $8 billion this year to deal with one of the world's most acute humanitarian crises, and the Brussels gathering responded with some fresh pledges of aid.
Hours before the UN Security Council meeting over a resolution proposed by Washington, London and Paris on the attack, Guterres said:
The three countries blamed Assad for the attack, possibly the third chemical attack in a month. The Syrian regime denies it.
Donors pledge $6B
A senior European Union official said international donors have pledged $6 billion to help conflict-torn Syria this year, a figure in line with their target.
EU Humanitarian Aid Commissioner Christos Stylianides said donors from more than 70 countries meeting in Brussels had made a "collective pledge of $6 billion for this year alone."
Stylianides said Syria's "needs are massive. Our conference is sending a powerful message. We are not letting down the people of Syria."
He described the pledge made at the conference as "an impressive figure. These commitments are significant."