sccm database sql transaction log grown too big

SCCM 2007 SP2 R2 central server has got DB (mdf) size 25GB and transaction log (LDF) 70GB. (SQL 2005 9.0.5000)

Backup tasks failed to backup LDF due to not enough free disk space available and this in turn makes LDF file grow further big.

Is there a way to trancate the transaction log and then perform a backup?

Thanks