Hillary Clinton, who steps down as U.S. secretary of state on February 1, says Iran and Russia continue to provide military and financial assistance to the Syrian government.
Clinton told reporters on January 31 that keeping Syrian President Bashar al-Assad in power was one of Iran's "highest priorities."
She added that Iran had increased the number of advisers to Damascus and improved the quality of weapons it provides.
Clinton said there were reasons to believe Russia continued to supply Assad with money and "equipment." She said that "the Russians are not bystanders in their support for Assad."
The outgoing secretary of state said it was "within the realm of the possible now" that the Syrian conflict spread into the neighboring countries.
She made the remarks to reporters following her final speech as secretary at the Council of Foreign Relations in Washington.
Clinton told the audience that the United States was "stronger at home and more respected in the world" than when she first took the top diplomatic post.
"A lot has changed in the last four years. Under President [Barack] Obama's leadership we've ended the war in Iraq, begun a transition in Afghanistan, and brought Osama bin Laden to justice," Clinton said.
"We have also revitalized American diplomacy and strengthened out alliances. And while our economic recovery is not yet complete, we are heading in the right direction. In short, America today is stronger at home and more respected in the world, and our global leadership is on firmer footing than many predicted."
Clinton said the State Department had begun to implement "21st-century tools" of diplomacy since she became its head in 2009, including new outreach to emerging powers and average citizens, such as through the use of new technologies and messaging.
China and Russia, she said, are fast implementing their own communication and outreach strategies suited to a more interconnected world.
While considering the last four years, Clinton also looked to the future.
She said the United States would remain the world's "indispensable power," but must "adapt to new realities of global power and influence."
Clinton's successor as secretary of state, U.S. Senator John Kerry, begins work on February 4.
Clinton said pressing foreign-policy goals for his tenure and beyond would include solidifying the U.S. position in Asia, finishing the war in Afghanistan, navigating the changes in the Middle East, and maintaining broad economic engagement.
She also called for U.S. leadership on climate change and for Washington to build on its legacy of defending human rights around the world.
Clinton described achieving equality for women around the world as "the unfinished business of the 21st century."