Defense Secretary Mark Esper said Monday during a trip to Afghanistan that the Pentagon is deliberating whether to station a “residual force” of U.S. soldiers in cities in eastern Syria to defend oil fields.

Why it matters: Stationing the residual force in a Syrian city would attach a major caveat to President Trump's pledge to stop U.S. involvement in “endless wars” in the Middle East.

What they're saying: Esper said the purpose of the force would be to "deny access, specifically revenue to ISIS and any other groups that might want to seek that revenue to enable their own malign activities."

However, the defense secretary said the proposal has not been presented to Trump.

Esper made the announcement as U.S. soldiers were arriving in eastern Iraq after withdrawing from northeastern Syria, where the Turkish military staged an offensive against Kurdish fighters earlier this month.

A U.S. official told the Washington Post that around 200 soldiers could secure the oil fields indefinitely.

Of note: In a tweet on Sunday, Trump quoted Esper on Syria, saying "U.S. soldiers are not in combat or ceasefire zone. We have secured the Oil." It's unknown when Esper said this.

The big picture: In addition to deterring the Islamic State, the force would also prevent Syria’s Russian- and Iranian-backed government, which is slowly recovering the territory it lost during the civil war, from reaping profits from the oil fields, a U.S. official told the Post.

Go deeper: State Department circulates talking points contradicting Trump on Syria