Obama hails Afghan security pact

US ambassador James Cunningham and Afghan security chief Mohmmad Hanif Atmar exchange documents in Kabul. (AP)

Barack Obama says the signing of a security agreement between the US and Afghanistan marks a "historic day" in the partnership between the two countries.

The pact, which allows about 10,000 US troops to stay in Afghanistan after the end of this year, was signed after a lengthy political struggle in Afghanistan that delayed its implementation.