World leaders are converging on the United Nations for the UN General Assembly's 70th anniversary meeting.
The gathering of 160 world leaders comes during what United Nations Secretary-General Ban Ki-moon has called "a time of turmoil and hope."
Syria will be a key issue, as world leaders seek an end to a conflict that has killed over 200,000 people and displaced millions more.
U.S. President Barack Obama and Russian President Vladimir Putin are among those scheduled to address the General Assembly on its first day on September 28.
It is Putin's first appearance at the General Assembly in a decade.
Putin is expected to focus on fighting Islamic extremism in the Middle East, including Syria where Russia has increased its military presence in recent weeks to support the regime of Bashar al-Assad, sparking concerns in Washington.
Analysts expect Obama to criticize Russia for its moves in Syria as well as eastern Ukraine, where Moscow is accused of directly backing pro-Russian rebels, something the Kremlin denies.
Iranian President Hassan Rohani will also address the General Assembly on September 28. He is expected to focus on the crisis in Syria.
On September 27, Rohani said fighting radical militants like Islamic State in Syria is the top priority and if they are to be defeated then Assad's government "can't be weakened."
"This does not mean that the Syrian government does not need reform ... Of course it does," Rohani told an audience of U.S. think-tanks and journalists on the sidelines of the General Assembly meeting in New York.
However, Rohani added that the removal of his ally Assad would turn Syria into an extremist safe haven.
Rohani will cut short his visit to New York and return to Tehran for the funeral of Iranians killed in the hajj tragedy in Saudi Arabia, Iran's state news agency IRNA reported on September 27.