The US has hailed a power-sharing deal between rival Afghan presidential candidates as a "moment of extraordinary statesmanship" that ended a prolonged dispute threatening to plunge the war-torn nation into further political turmoil and complicate the NATO troops withdrawal.
"The inauguration of the new President, appointment of his Chief Executive, and the signing of the Bilateral Security Agreement and NATO SOFA will open a new chapter in our enduring partnership with Afghanistan," said US Secretary of State John Kerry.
He said: "The US remains determined to honour the Afghan people's historic achievement by helping their transition succeed."
Ashraf Ghani and Abdullah Abdullah signed a deal breaking a three-month deadlock over allegations of massive electoral fraud that plunged Afghanistan into crisis as US-led troops end their 13-year war against the Taliban.
Both Ghani and Abdullah claimed to have won the vote.
Under the agreement Ghani would become the new president, while Abdullah would be the chief executive officer (CEO) -- which will be similar to prime minister.
Kerry, who first got Ghani and Abdullah to agree in principle to share power, said the two leaders have put the people of Afghanistan first, and they have ensured the first peaceful democratic transition in the history of their country begins with national unity.
"In the days to come, Afghanistan has an enormous opportunity to grow stronger from this recent moment of testing. Elections are not the end," he said.
"They must be the beginning, where Afghanistan and its people move forward on a reform agenda and make improvements to the electoral process."
In a separate statement, White House Press Secretary Josh Earnest applauded Abdullah and Ghani for signing the deal.
"Signing this political agreement helps bring closure to Afghanistan's political crisis, and restores confidence in the way forward," said Earnest.
"This agreement marks an important opportunity for unity and increased stability in Afghanistan. We continue to call on all Afghans, including political, religious, and civil society leaders, to support this agreement and to come together in calling for cooperation and calm," he said.
The new administration will have to stabilise the economy and deal with worsening unrest.
Nearly 41,000 NATO troops remain in Afghanistan fighting the fierce Taliban insurgency alongside Afghan soldiers and police.
The combat mission of NATO will end in December, with a follow-on force of about 12,000 troops likely to stay into 2015.
