WASHINGTON — The United States banned the export of weapons and defense services to South Sudan on Friday in a reflection of the Trump administration’s growing frustration over that nation’s grinding civil war.

The ban has little practical effect, since the United States does not sell weapons to the country. But the announcement on Friday was the first step in a broader effort to cut off weapons to a conflict that has put 1.5 million people on the brink of starvation.

Heather Nauert, the State Department’s spokeswoman, said the administration would soon resume a push begun under President Barack Obama for the United Nations Security Council to impose a global arms ban against South Sudan.

“The message must be clear: The United States, the region and the international community will not stand idly by as innocent South Sudanese civilians are murdered,” Ms. Nauert said.