US Defense Secretary Jim Mattis says the US military's policy in Syria has not changed and emphasized that its priority is the defeat of the Islamic State militant group. <br/><br/>Speaking at his first news conference since last week's US missile attack on a Syrian airbase,Mattis said Washington had determined that a measured military response could best deter the Syrian regime from using chemical weapons again. He warned that Syria will pay a very stiff price for any future use of chemical weapons.<br/>Syria denies involvement in the chemical weapons attack on an opposition-held territory that killed at least 72 people. Russia, which backs Syria, has criticized the US missile attack.