US President Barack Obama urged Yemen to immediately implement a deal under which President Ali Abdullah Saleh has agreed to hand over power after 33 years in office.
"The United States will continue to stand by the Yemeni people as they embark on this historic transition," Obama said in a written statement.
Saleh, who has been the target of opposition protests since January, signed the deal in Riyadh, ending months of delay that had seen protests degenerate into deadly unrest.
Under the agreement, the veteran leader will hand over his powers to Vice President Abdrabuh Mansur Hadi but remain honorary president for 90 days.
"The United States urges all parties to move immediately to implement the terms of the agreement, which will allow Yemen to begin addressing an array of formidable challenges and chart a more secure and prosperous path for the future," Obama said.
He praised the Yemeni people for "courageously and steadfastly" pressing for change in their country despite "violence and extreme hardship."
"Today marks a significant step forward for the Yemeni people in their quest for a unified, democratic, secure, and prosperous Yemen," said Obama's top diplomat Hillary Clinton, praising neighboring Gulf states for their role in brokering the deal
"We urge all parties within Yemen to refrain from violence and to move swiftly to implement the terms of the agreement in good faith and with transparency -- including credible presidential elections within 90 days."
She said Washington would continue to "closely monitor" the political transition in Yemen, and looked forward to shoring up ties with Sanaa.
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Saleh had repeatedly backed out of signing the deal brokered by Yemen's wealthy Gulf neighbors since the parliamentary opposition inked it back in April.
During his months of prevarication, deadly clashes between loyalist and dissident troops had riven the capital, while militants, some linked to Al-Qaeda, took advantage of the decline of central government control in the provinces to set up base.
King Abdullah of Saudi Arabia, who hosted the signing ceremony at his Al-Yamama palace in Riyadh, hailed a "new page" for the impoverished neighbouring country, while US President Barack Obama called on Yemenis to immediately implement the "historic transition" that had been agreed.
Saleh promised "real partnership" with the opposition in implementing the Gulf- and UN-brokered agreement, but thousands of demonstrators again to took to the streets of the capital Sanaa to protest against the deal's promise of immunity from prosecution for both him and his family.
Spokesman Walid al-Amari told AFP they rejected the agreement and called for further mass demonstrations.
The 69-year-old Saleh sustained serious blast wounds in a June bombing of his residence for which he has already received extensive treatment in Saudi Arabia.
Under the roadmap brokered by Benomar, Saleh will hand to Hadi "all powers necessary for proceeding with the Gulf initiative and its implementation mechanism and for organising early elections within a 90-day period which begins immediately after the signing."
Saleh will remain honorary president for 90 days until Hadi is elected as consensus president for an interim two-year period.
The parliamentary opposition, which co-signed the agreement in Riyadh, will put forward a candidate to head a government of national unity, that will be charged with holding talks with the youth activists who have spearheaded the 10 months of protests, the UN envoy said.
But an opposition spokesman warned that the protesters were unlikely to give up their almost daily demonstrations until Saleh has finally quit office completely.
"People will not go back to their homes until the honorary term ends," Mohammed Qahtan, spokesman for the Common Forum opposition bloc, told AFP.