Facing issue in connecting with ssl soap service

Hi All,
I am currently facing issue in connecting to a soap https service from soapui. I am provided with public and private certificates. I have added the public certificate in the preferences--> SSL setting tab. And then I have configured the outgoing ws security --> keystore with the private key. Also I have created a signature with the private certificate.
When I hit the service am getting a soap response saying "certificate is not trusted" . Am new to this setup and please correct me if am wrong.Can you please help me in resolving the issue ?