BBC News
Boris Johnson is to urge the international community to accept the Syria conflict will not end while President Assad remains in power.
The foreign secretary is hosting talks in London later with his US, French, German and Italian counterparts.
He said Russia must unite behind calls for the Syrian leader to step down.
Meanwhile, Mr Johnson and US Secretary of State John Kerry are to hold discussions for the first time following the UK vote to leave the EU.
The talks are being seen as an attempt by the government to show that the US still values the "special relationship" between the two countries after the referendum.
Mr Johnson is also holding a meeting with foreign ministers from Saudi Arabia and the UAE to discuss the conflict in Yemen.
Speaking on Monday, Mr Johnson said: "We must be more active, more engaged and more outward-looking, so I am delighted to have this early opportunity to welcome my international counterparts to London for important meetings on the conflicts in Syria and Yemen.
"I will be making clear my view that the suffering of the Syrian people will not end while Assad remains in power. The international community, including Russia, must be united on this."
EU foreign affairs chief Federica Mogherini will also participate in the talks in London on Syria.
BBC diplomatic correspondent James Landale says Mr Johnson's remarks will be seen as the latest attempt to distance himself from his past views and colourful language as he steps tentatively onto the world stage.
He has in the past written newspaper columns suggesting the West should work with President Bashar al-Assad to fight IS and has praised the Syrian leader's success in ousting the group from the historic ruins of Palmyra.
But our correspondent said the latest comments show the foreign secretary, who was appointed last week by new Prime Minister Theresa May, is backing official government policy.
Peace talks on Syria, co-sponsored by the US and Russia, have not formally examined whether any deal could require President Assad's departure from the country.
But British officials denied Mr Johnson was sending a wider signal to the US that the UK would oppose any moves towards an accommodation with President Assad in the future.
Mr Johnson, who made his debut on the international stage in Brussels on Monday when he met EU foreign ministers, will travel to Washington on Thursday for talks on combating IS.