Bashar Assad must not escape prosecution even if he is allowed to play an interim role in any political solution in Syria, David Cameron insisted as he arrived in the US.
Injecting new momentum into the stalled peace process is top of the agenda for world leaders gathered in New York for the 70th anniversary meeting of the United Nations General Assembly.
Vladimir Putin is among those attending amid concerns over an unexpected build-up of Russian troops in support of the Moscow-backed Assad regime, which the Kremlin says is the best way to take on Islamic State fighters there.
The Prime Minister will use one-to-one talks with a number of key figures - though not Mr Putin - to press his case that the Syrian president "can't be part" of a peaceful solution to the civil war.
But with Europe increasingly overwhelmed by the upsurge of refugees fleeing the conflict and IS - also known as Isil - in control of large swathes of the country, he joined Western allies such as the US and France in signalling a willingness to discuss whether Assad could play a transitional role.
He said Russia and other powers had to be persuaded that it was "in everyone's interests" to co-operate in the fight against IS. The president of Iran - which also backs Assad - is among other leaders at the UN.