Under the deal, the insurgents gain power sharing and amnesty for war crimes, but the United Nations says that immunity will end at the nation's borders.
The agreement is the second in three years aimed at ending a conflict that has killed tens of thousands of people. Many others have been maimed, kidnapped or raped and more than half of the West African country's 4.7 million people are displaced.
I shall sign it as president of Sierra Leone, but more importantly I shall sign it for the thousands of children of Sierra Leone, Kabbah said.
Shortly before signing, Kabbah lifted high 2-year-old Maimouna Massira, whose right arm had been hacked off by rebel fighters. The girl clutched a teddy bear with her remaining arm.
This is the product of war, Kabbah said to roars of agreement from the mainly Togolese and Sierra Leonean crowd. I hope we shall all learn from this and try to embrace peace.
Sankoh, clad in shiny white robes, was silent for most of the ceremony. But after the signing, he apologized for rebel war crimes.
Let us try to forgive. We are asking for forgiveness. We need the support of everyone, especially our brother the president, he said.
But U.N. officials, under pressure from human rights groups, refused to recognize the peace agreement's amnesty clause for war crimes.
The U.N. Special Representative for Sierra Leone, Francis Okelo, signed the agreement with a disclaimer saying the amnesty does not cover human rights violations, spokesman Manoel de Almeida e Silva said in New York.
While any sovereign state may grant amnesty for violation of its national laws ... the United Nations will not recognize that amnesty as applying to gross violations of human rights, de Almeida e Silva said.
He would not say, however, what the United Nations would do to make sure those who committed atrocities are punished.
The presidents of Nigeria, Burkina Faso, Liberia and Togo signed the accord as witnesses. But questions remained as to whether the peace deal would be accepted by fighters and civilians.
A 1996 peace deal collapsed after several months when rebel and military forces toppled Kabbah's government. The bloodletting that followed was worse than at any time since the war began in 1991. Kabbah returned to power last March.
In a long-expected move, Kabbah gave Sankoh amnesty. The rebel leader had been sentenced to death for treason last October.
In a written statement released while he visited South Dakota, President Clinton said the agreement offered the hope of ending a terrible conflict in Sierra Leone and bringing peace and a brigher future for its people.
Both the United States and Britain are closely involved in the peace process in Sierra Leone and had envoys who observed the month-long negotiations and witnessed the peace accord.
©1999 CBS Worldwide Corp. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press and Reuters contributed to this report