JUDY WOODRUFF: The White House today reinforced a stark warning to the government of Syria's Bashar al-Assad against using chemical weapons.

Last night, the administration announced that the U.S. had detected — quote — "potential preparations" by the regime to use the banned weapons, again.

Chief foreign affairs correspondent Margaret Warner reports.

MARGARET WARNER: President Assad was seen today touring a Russian air base just hours after last night's White House statement. It warned that the Syrian regime would pay a heavy price for launching another chemical attack.

Today, the Pentagon elaborated, saying the U.S. has seen active preparations for chemical weapons use at Shayrat air base near Homs in Western Syria.

Nancy Youssef is senior national security correspondent at BuzzFeed.

NANCY YOUSSEF, BuzzFeed: The Pentagon told us that they saw evidence of planes that were used to conduct aerial chemical attacks in the past being moved around on Shayrat airfield. We also heard reports of chatter happening on the ground in preparation for a possible imminent attack. The number of people who knew was so limited, and the message from the White House was so ominous, that it created a real confusion that we are just not used to.

MARGARET WARNER: A chemical weapons attack was launched from that same base in April, killing dozens of men, women and children. The Syrians denied responsibility. But President Trump ordered the U.S. military to fire 59 Tomahawk missiles at the base.

Mr. Trump's response was a sharp departure from his predecessor's caution about intervening in Syria's civil war. In 2012, President Obama famously warned that the use of chemical weapons use would constitute a red line, triggering U.S. action. But instead, after a deadly August, 2013, attack on a Damascus suburb, then-Secretary of State John Kerry and Russia worked on a deal with Assad that aimed to rid his regime of chemical weapons.

Trump officials say that strategy clearly didn't work, and it's taking a tougher approach to Syria and its key backers.

U.N. Ambassador Nikki Haley spoke at a House hearing today.

NIKKI HALEY, U.S. Ambassador to the United Nations: I believe that the goal is, at this point, not just to send Assad a message, but to send Russia and Iran a message that, if this happens again, we are putting you on notice.

MARGARET WARNER: A spokesman for Russian President Vladimir Putin retorted: "Such threats to Syria's legitimate leaders are unacceptable."

Iran's foreign minister said: "Another dangerous U.S. escalation in Syria on fake pretext will only serve ISIS," the Islamic State group.

Since the April 3 U.S. missile barrage, the U.S. has had several confrontations with pro-Assad forces, shooting down a Syrian air force jet and an armed Iranian-made drone. In response, Russia has threatened to target coalition aircraft in certain parts of Syria. Today, it's unclear what the broader U.S. strategy is.

NANCY YOUSSEF: Is the U.S. committed to a form of deterrence against the regime for possible chemical attacks? And, if so, in what form? Going forward, will the U.S. put out a statement every time it suspects chemical weapons attacks? Is that what we are to take away from yesterday's statement?

MARGARET WARNER: For the PBS NewsHour, I'm Margaret Warner.