Mr Al-Sissi thanked Mr Trump in a televised speech on Monday for helping to initiate a ceasefire after “two years of war, genocide, killing, and destruction.”
He warned that institutions fail “not for lack of noble ideals but when they drift into irrelevance”.
He called for an immediate ceasefire in Gaza and a return to negotiations to end the war.
Meanwhile, critics warn the pressure on Israel could further escalate tensions. Israel and its ally, the United States, have boycotted the conference.
Some rail connections were delayed by more than an hour.
The UK will use the UN General Assembly to push for international backing of a framework for peace in the Middle East, following its recognition of a Palestinian state.
The victims were reported to include a pregnant woman.
The UN chief made a blunt appeal, saying, “The United Nations is the place. Next week is the time. Leaders must get serious – and deliver.”
The UN is “underfunded, overstretched and under attack”, Tom Fletcher, head of the UN Office for the Coordination of Humanitarian Affairs, has said.
