ISTANBUL — President Recep Tayyip Erdogan on Tuesday fended off an international backlash over Turkey’s offensive in northeast Syria that has wrought US economic sanctions and a widening arms embargo from Europe as fighting with Kurdish militants entered its seventh day.

Russia stepped into the lurch left by the United States’ about-face last week to abandon its Kurdish partners in Syria. Russian fighters Tuesday took over a US base and began patrolling a new frontline between Turkey and the Syrian government army that Moscow backs after the Kurdish-dominated Syrian Democratic Forces (SDF) turned to Damascus to ward off the Turkish assault.

President Donald Trump’s decision to withdraw US troops after a phone call with Erdogan has reconfigured the battle lines in an eight-year war that has already claimed hundreds of thousands of lives. In one week, Trump’s reversal of US policy has allowed Syrian President Bashar al-Assad to send troops to areas they left in 2012, severely tested Turkey’s ties with its traditional Western allies and expanded Russian clout in the region.

For Turkey, continuing the offensive despite condemnation from Western governments was “a struggle for survival. … We will maintain our struggle until we reach our objective,” Erdogan said during a visit to Azerbaijan. Turkey sees the SDF as a grave national security threat because of its ties to a decadeslong Kurdish insurgency within Turkey.