An ISAF spokeswoman in Kabul, the Afghan capital, said: "There was an explosion in the vicinity of Salerno early this morning in Khost province. There were no ISAF casualties. However we are aware of a number of other casualties."
Afghan police could not confirm the toll.
Civilian victims
No-one immediately claimed responsibility for the blast.
Zemarai Bashary, a police spokesman in Kabul, said: "The suicide bombing was in front of a base of the coalition forces."
He said he was unable to confirm casualties.
Foreign troops cordoned off the area about a kilometre from the blast site and did not allow anyone to enter.
There were nearly 140 such attacks in the country last year,  according to US officials.
Most were aimed at foreign and Afghan forces, but killed more civilians.