US Secretary of State Antony Blinken held out the assurance in a phone conversation with President Ashraf Ghani on on Friday.
The US State Department said the two sides exchanged views on regional efforts to advance the Afghan peace process.
Recent incidents of violence, including the attack on Syed Al-Shuhada School that killed more than 60 students, also came up for discussion.
A statement from the State Department said Blinken conveyed Eid greetings to the president and extended his deepest condolences to the families of victims of recent violence.
Blinken and Ghani stressed the need for unity among the Afghans and a regional push to promote the peace process in Afghanistan.
Following the phone call, the secretary of state tweeted: “We support our partnership and Afghan security forces.”