"The co-conveners are committed to supporting a sovereign, independent and unified Afghanistan," they said in a joint statement on Tuesday.
Known officially as the Istanbul Conference on the Afghan Peace Process, the talks are due to run through May 4.
The overriding objective of the Istanbul Conference on the Afghanistan Peace Process is to accelerate and complement the ongoing intra-Afghan negotiations in Doha on the achievement of a just and durable political settlement.
Participation in the Conference and its agenda have been the subject of extensive consultations with the Afghan parties.
The Conference will focus on helping the negotiating parties reach a set of shared, foundational principles that reflect an agreed vision for a future Afghanistan, a roadmap to a future political settlement and an end to the conflict.
"It is our expectation that the Conference will provide an important opportunity for all partners to reiterate support for the people of Afghanistan on their path toward inclusive peace, stability, and prosperity," they stated.