WASHINGTON - The US strike on a Syrian air base destroyed a fifth of the Damascus regime's remaining warplanes, Pentagon chief Jim Mattis said Monday as Washington fired a fresh salvo of warnings at President Bashar al-Assad.

The public assessment of Friday's missile strike and the forceful rhetoric came as G7 ministers met in Italy to send a "clear and coordinated" message to Russia over its support for Damascus.

The United States on Friday blasted 59 Tomahawk missiles at the Shayrat air base near Homs, which the Pentagon said Assad's jets had used to launch a deadly chemical attack on rebel-held Idlib province.

"The United States will not passively stand by while Assad murders innocent people with chemical weapons, which are prohibited by international law and which were declared destroyed," Mattis said in a statement, noting that 20 percent of Assad's "operational" aircraft were destroyed.

"The Syrian government would be ill-advised ever again to use chemical weapons," he added.

Mattis's warning came as White House spokesman Sean Spicer appeared to lower the threshold for new US action against Assad to include barrel bombs, a crude yet hugely destructive weapon of choice for the Syrian leader.

"If you gas a baby or drop a barrel bomb onto innocent people, you will see a response from" President Donald Trump, Spicer said.

But US officials later walked back Spicer's remarks.

"Nothing has changed in our posture," a senior administration official said.

"As the president has repeatedly made clear, he will not be telegraphing his military responses," the official said.

Trump discussed Syria during separate telephone calls Monday with British Prime Minister Theresa May and German Chancellor Angela Merkel.

May and Merkel "expressed support for the action of the United States and agreed with President Trump on the importance of holding Syrian President Bashar al-Assad accountable," the White House said in a statement.

Downing Street said Trump and May had "agreed that a window of opportunity now exists in which to persuade Russia that its alliance with Assad is no longer in its strategic interest."

The two leaders were looking to US Secretary of State Rex Tillerson's trip to Moscow this week as an opportunity to lay the groundwork for a "lasting political settlement," May's office said.

- 'Toxic' Assad -

At the outset of the G7 gathering in the Tuscan city of Lucca, Britain's Foreign Secretary Boris Johnson described Assad as "toxic," and said it was "time for (Russian President) Vladimir Putin to face the truth about the tyrant he is propping up."

Tillerson also attended the meeting at the 15th century Ducal Palace, along with foreign ministers from Canada, France, Germany, Italy and Japan.

The annual two-day meeting had initially been expected to focus on talks with Tillerson about hotspots like Libya, Iran and Ukraine.

But the agenda is now likely to be dominated by last week's suspected chemical weapons attack that killed at least 87 civilians.

Washington's missile strike was the first time it had intervened directly against the regime of Assad, who is fighting a civil war with the backing of Russia and Iran.

Several rounds of UN-backed peace talks have failed to end the conflict, which has killed more than 320,000 people since March 2011.

   - 'End Assad support' -

Iran and North Korea have slammed Washington's retaliation and put it on a direct diplomatic collision course with Moscow, where Tillerson heads Tuesday for talks with Russian counterpart Sergei Lavrov.

Johnson on Monday called on Moscow to do "everything possible to bring about a political settlement in Syria and work with the rest of the international community to ensure that the shocking events of the last week are never repeated."

French Foreign Minister Jean-Marc Ayrault said the pressing task for the G7 was to "find a political solution, a political transition" in Syria, particularly if the West wanted to triumph over the Islamic State group.

  - 'Crime against innocents' -

Italy arranged a last-minute meeting for Tuesday between the G7 ministers and their counterparts from Jordan, Qatar, Saudi Arabia, Turkey and the United Arab Emirates.

Italian media said the aim was "to avert a dangerous military escalation."

Japan's Foreign Minister Fumio Kishida said he told Tillerson that Tokyo supports the US in its push to "deter the spread and use of chemical weapons," and discussed the pressing North Korean nuclear threat.

Japan hopes the strong US response on Syria will also put pressure on Pyongyang, which is showing signs of preparing for its sixth nuclear test and more test-firings of ballistic missiles.

"We agreed that the role of China is extremely important. Japan and the United States will jointly call on China to play a bigger role," Kishida told reporters after meeting Tillerson in Lucca.

Meanwhile, the leaders of southern EU nations said Monday the US missile strike on the Syrian air base was "understandable."

"The strike launched by the United States on Shayrat Airfield in Syria had the understandable intention to prevent and deter the spread and use of chemical weapons and was limited and focused on this objective," they said in a joint statement after a summit in Madrid.-AFP