WASHINGTON — Allied missiles struck at the heart of Syrian chemical weapons arsenal in a show of force and resolve aimed at punishing the Assad government for a suspected poison gas attack against civilians and deterring the possible future use of such banned weapons.

"A perfectly executed strike," President Donald Trump tweeted Saturday in the aftermath of his second decision in two years to fire missiles against Syria. "Could not have had a better result. Mission Accomplished!"

His choice of words recalled a similar claim associated with President George W. Bush following the U.S.-led invasion of Iraq. Bush addressed sailors aboard a ship in May 2003 alongside a "Mission Accomplished" banner, just weeks before it became apparent that Iraqis had organized an insurgency that tied down U.S. forces for years.

Syria's chief allies, Russia and Iran, called the use of force by the United States, Britain and France a "military crime" and "act of aggression" with the potential to worsen a humanitarian crisis after years of civil war. The U.N. Security Council planned to meet later Saturday at Moscow's request.

"Good souls will not be humiliated," Syrian President Bashar Assad tweeted, while hundreds of Syrians gathered in Damascus, the capital, where they flashed victory signs and waved flags in scenes of defiance after the one-hour barrage launched Friday evening (early Saturday in Syria).

Lt. Gen. Kenneth McKenzie, the director of the Joint Staff at the Pentagon, said no aircraft or missiles involved in the operation "were successfully engaged by Syrian air defenses." He said 105 weapons were launched against three targets, and that the U.S. was not aware of any civilian casualties.

A global chemical warfare watchdog group said its fact-finding mission would go as planned in Douma, where the apparent use of poison gas against civilians on April 7 that killed more than 40 people compelled the Western allies to launch their attack. Syria has denied the accusation.

But France's foreign minister said there was "no doubt" the Assad government was responsible, and he threatened further retaliatory strikes if chemical weapons were used again, as did Pentagon chief Jim Mattis, who said the assault was a "one-time shot," as long as chemical weapons weren't used again.

NATO representatives planned a special session to hear from U.S., British and French officials.

Pentagon officials said the attacks, carried out by manned aircraft and from ships that launched cruise missiles from the Mediterranean Sea, targeted the heart of Assad's programs to develop and produce chemical weapons, and delivered "a very serious blow," said McKenzie.

Trump said the U.S. was prepared to sustain economic, diplomatic and military pressure on Assad until he ends what Trump called a criminal pattern of killing his own people with internationally banned chemical weapons. That did not mean military strikes would continue; in fact, Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said no additional attacks were currently planned.

Russian leader Vladimir Putin reaffirmed the Kremlin's skepticism about the allies' Douma claim, saying Russian military experts had found no trace of the attack. He criticized the U.S. and its allies for launching the strike without waiting for international inspectors to visit the area.

But British Prime Minister Theresa May cited reports she said indicated the Syrian government used a barrel bomb — large containers packed with fuel, explosives and scraps of metal — to deliver the chemicals. "No other group" could have carried out that attack, she said, adding that the allies' use of force was "right and legal."

German Chancellor Angela Merkel said the West's response was "necessary and appropriate."

Mattis disclosed that the U.S. had not yet confirmed that the Douma attack — the most recent suspected Syrian chemical weapons attack, on April 7 — included the use of sarin gas. He said at least one chemical was used: chlorine, which also has legitimate industrial uses and had not previously triggered a U.S. military response.

He said the targets selected by U.S., British and French officials were meant to minimize civilian casualties.

"This is difficult to do in a situation like this," he said, in light of the volatility of chemical agents.

Defense officials from the countries involved in the attack gave differing accounts of how much warning was given to the Russians, Syria's powerful ally.

Dunford said the U.S. did not coordinate targets with or notify the Russian government of the strikes, beyond normal airspace "de-confliction" communications. But the description from an ally put things differently. French Defense Minister Florence Parly said that "with our allies, we ensured that the Russians were warned ahead of time."

At a Pentagon news conference alongside Mattis, and with British and French military officers beside them to emphasize allied unity, Dunford said the attacks targeted mainly three targets in western Syria.

Dunford said missiles first struck a scientific research center in the Damascus area that he said was a center of Syrian research, development, production and testing of chemical and biological warfare technology. The second target was a chemical weapons storage facility west of Homs. He said this was believed to be the main site of Syrian sarin and precursor chemical production equipment.

The third target was a chemical weapons equipment storage facility and an important command post, also west of Homs, Dunford said.

British leader May said in London that the West had tried "every possible" diplomatic means to stop Assad from using chemical weapons. "But our efforts have been repeatedly thwarted" by Syria and Russia, she said.

"So there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime," May said. "This is not about intervening in a civil war. It is not about regime change."

French President Emmanuel Macron said in a statement that a target of the strike was the Syrian government's "clandestine chemical arsenal."

The Syrian government has repeatedly denied any use of banned weapons.

The decision to strike, after days of deliberations, marked Trump's second order to attack Syria. He authorized a barrage of Tomahawk cruise missiles to hit a single Syrian airfield in April 2017 in retaliation for Assad's use of sarin gas against civilians.

Trump chastised Russia and Iran for supporting "murderous dictators," and noted that Putin had guaranteed a 2013 international agreement for Assad to get rid of all of his chemical weapons.

White, the Defense Department spokeswoman, said the strikes did not "represent a change in U.S. policy or an attempt to depose the Syrian regime." But, she said, "We cannot allow such grievous violations of international law."