Hilary Benn has warned the Syrian civil war is the "great humanitarian crisis of our age", amid a shadow cabinet split over whether to back air strikes against Isis in the country.
David Cameron is expected to ask parliament to approve the expansion of Britain's bombing campaign against Isis from Iraq into Syria.
While Corbyn has expressed deep misgivings about new military strikes, a substantial number of his MPs want to support the prime minister. Several shadow cabinet minister are also seen as being in favour.
Corbyn has been under pressure to grant his MPs a free vote when it is brought to the Commons, allowing those in favour of strikes to vote to approve strikes without formally splitting the party.
In his speech, Benn did not indicate how the party leadership would approach the vote.
Diane Abbott, the new shadow international development secretary, told the conference: "I will be voting against bombing Syria." While shadow justice secretary Lord Falconer has previously said strikes "should" take place provided there was a proper legal basis.
Benn told Labour's conference in Brighton on Monday morning that each death in the Syrian conflict was "a rebuke to the world" and criticised the international community as "no one has taken responsibility".
The shadow foreign secretary said the world had a "moral duty to act" to try and bring the conflict to an end.
In New York, Cameron is due hold face-to-face talks with Iranian president Hasan Rouhani in a fresh bid to revive the stalled Syrian peace process. Both leaders are in the United States for a United Nations summit.
The prime minister will use one-to-one talks with a number of key figures - though not Russian president Vladimir 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 in control of large swathes of the country, he has joined Western allies such as the US and France in signalling a willingness to discuss whether he could play a transitional role.
- A poll by Survation for The Huffington Post UK showed today a majority of Labour voters do not believe Jeremy Corbyn should apologise for the Iraq War