LONDON (Reuters) - New British Foreign Secretary Boris Johnson called on Tuesday for Russia and other states to demand Syrian President Bashar al-Assad step down as he prepared for his first meeting with Western counterparts on Syria's intractable civil war.

Johnson will meet peers from Germany, France, Italy and the European Union in London to discuss Syria's five-year conflict, which has spurred the rise of Islamic State, sucked in regional and major powers and created the world's worst refugee crisis.

"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," Johnson will say, according to remarks released by his office in advance of the meeting.

Johnson will also see U.S. Secretary of State John Kerry in London on Tuesday, less than a week after the ex-London mayor known for his undiplomatic wit was surprisingly appointed as foreign secretary by incoming Prime Minister Theresa May.

Russia and the United States are co-sponsors of peace talks, and back opposing sides, in the Syria conflict. Those talks, currently on hold, have so far carefully skirted the question of whether a peace deal would require Assad’s departure, so negotiations could theoretically limp along despite contradictions between the stances of Moscow and Washington.

Sources familiar with the Kremlin's thinking said in June that Russia will countenance Assad leaving office, but only when it is confident a change of leader will not trigger a collapse of the Syrian government.

The last round of talks between the Syrian government and opposition broke up at the end of April as Assad's forces, backed by Russian air strikes, escalated their assault on rebel-held areas in the northern city of Aleppo.

Speaking in December, before his appointment, Johnson had called for Britain to set aside a "Cold War mindset" when dealing with Russia over Syria.

In a newspaper column in March this year, Johnson said "bravo for Assad" for saving the ancient city of Palmyra, a UNESCO World Heritage site, from Islamic State militants.

"...No matter how repulsive the Assad regime may be – and it is – their opponents in Islamic State ...are far, far worse," Johnson wrote.

(Reporting by William James; editing by Mark Heinrich)