PALM BEACH, Fla. — The United States vowed Friday to keep the pressure on Syria after the intense nighttime wave of missile strikes from U.S. ships, despite the prospect of escalating Russian ill will that could further inflame one of the world’s most vexing conflicts.

Standing firm, the Trump administration signaled new sanctions would soon follow the missile attack, and the Pentagon was even probing whether Russia itself was involved in the chemical weapons assault that compelled President Donald Trump to action. The attack against a Syrian air base was the first U.S. assault against the government of President Bashar Assad.

Much of the international community rallied behind Trump’s decision to fire the cruise missiles in reaction to this week’s chemical weapons attack that killed dozens of men, women and children in Syria. But a spokesman for Russian President Vladimir Putin warned that the strikes dealt “a significant blow” to relations between Moscow and Washington.

At the United Nations, Russia’s deputy ambassador, Vladimir Safronkov, strongly criticized what he called the U.S. “flagrant violation of international law and an act of aggression” whose “consequences for regional and international security could be extremely serious.” He called the Assad government a main force against terrorism and said it deserved the presumption of innocence in the chemical weapons attack.

U.S. officials blame Moscow for propping up Assad.

“The world is waiting for the Russian government to act responsibly in Syria,” Nikki Haley, the U.S. ambassador to the U.N., said during an emergency Security Council session. “The world is waiting for Russia to reconsider its misplaced alliance with Bashar Assad.”

Haley said the U.S. was prepared to take further action in Syria but hoped it wouldn’t be necessary.

In Florida with the president, meanwhile, Treasury Secretary Steven Mnuchin said: “We will be announcing additional sanctions on Syria as part of our ongoing effort to stop this type of activity and emphasize how significant we view this. We expect that those will continue to have an important effect on preventing people from doing business with them.”

Thursday night’s strikes — some 60 cruise missiles fired from two ships in the Mediterranean — were the culmination of a rapid, three-day transformation for Trump, who has long opposed deeper U.S. involvement in Syria’s civil war. Advisers said he was outraged by heartbreaking images of young children who were among the dozens killed in the chemical attack and ordered his national security team to swiftly prepare military options.

The decision undercut another campaign promise for Trump: his pledge to try to warm relations with Moscow. After months of allegations of ties between his election campaign and the Kremlin — the subject of current congressional and FBI investigations — Trump has found himself clashing with Putin.

On Friday, senior U.S. military officials were looking more closely at possible Russian involvement in the poison attack. Officials said a drone belonging to either Russia or Syria was seen hovering over the site after the assault earlier this week. The drone returned late in the day as citizens were going to a nearby hospital for treatment. Shortly afterward, officials say the hospital was targeted.

The officials, who insisted on anonymity in order to discuss the sensitive matter, said they believe the hospital attack may have been an effort to cover up evidence of the earlier assault.

White House officials caution that Trump is not preparing to plunge the U.S. deeper into Syria. Spokesman Sean Spicer said the missile attack sent a clear message to Assad, but he avoided explicitly calling for the Syrian to leave office.

“The president believes that the Syrian government, the Assad regime, should at the minimum agree to abide by the agreements they made to not use chemical weapons,” Spicer said when asked if Assad should step down.

The impact of the strikes was also unclear. Despite intense international pressure, Assad has clung to power since a civil war broke out in his country six years ago, helped by financial and military support from both Russia and Iran. Russian military personnel and aircraft are embedded with Syria’s, and Iranian troops and paramilitary forces are also on the ground helping Assad fight the array of opposition groups hoping to topple him.

Trump spent Friday in Florida, in private meetings with visiting Chinese President Xi Jinping. U.S. officials noted that the timing of the strike had the possible added benefit of signaling to China that Trump is willing to make good on his threat to act alone to stop North Korea’s nuclear pursuits if Beijing doesn’t exert more pressure on Pyongyang.

The missile strikes hit the government-controlled Shayrat air base in central Syria, where U.S. officials say the Syrian military planes that dropped the chemicals had taken off. The missiles targeted the base’s airstrips, hangars, control tower and ammunition areas, officials said.

Trump’s decision to strike Syria won widespread praise from other nations, including Saudi Arabia and Turkey, which support the Syrian opposition. British Prime Minister Theresa May’s office said the action was “an appropriate response to the barbaric chemical weapons attack launched by the Syrian regime, and is intended to deter further attacks.” France, Italy and Israel also welcomed the strikes.

Not everyone was cheering in Washington, where the president’s decision to act without congressional authority angered a mix of libertarian Republicans, Democrats and the far right.

“The Constitution is very clear that war originates in the legislature,” said Sen. Rand Paul, R-Ky., a leader of the party’s non-interventionist wing who challenged Trump for the GOP nomination. “You vote before you go to war, not after you go to war.”

Still, most Republican leaders applauded the president, and some Democrats backed him, too.

Sen. Dianne Feinstein of California said the strikes were “a limited but necessary response” and called on Trump to “develop a comprehensive strategy to end Syria’s civil war.”