The United States, Britain and France launched punitive military strikes against Bashar al-Assad's Syrian regime in response to its latest alleged chemical weapons atrocity, President Donald Trump announced Friday.

Shortly after Trump began a White House address to announce the action, large explosions were heard in the Syrian capital Damascus, signalling a new chapter in a brutal seven-year-old civil war.

"A short time ago, I ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad," Trump said in a primetime televised speech to the nation.

"A combined operation with the armed forces of France and the United Kingdom is now under way. We thank them both. This massacre was a significant escalation in a pattern of chemical weapons use by that very terrible regime."

A US official said the strikes had targeted chemical production facilities. Another official said multiple types of bombs were used, and a variety of targets chosen.

Trump also warned Russia and Iran not to stand by their ally in Damascus.

"Russia must decide if it will continue down this dark path or if it will join with civilized nations as a force for stability and peace," he argued.

The strikes had been expected since harrowing footage surfaced of the aftermath of the alleged toxic gas attack in the Damascus suburb of Douma, which took place a week ago, and Trump reacted in an emotional tweet.

"Many dead, including women and children, in mindless CHEMICAL attack in Syria," he declared.

"President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay."

Trump's anger and apparent determination was quickly matched by France's President Emmanuel Macron, who signed his country up for a joint response.

Israeli air strikes have killed four Palestinians in Gaza as European governments sought UN action to end more than six weeks of bloodshed.

Fighting flared anew on Tuesday as Egyptian-brokered truce efforts collapsed, with Israel insistent on its demand for security from rocket fire by Gaza militants, and Hamas defiant over its call for an end to eight years of Israeli blockade.

The death toll since July 8 now stands at least 2087 Palestinians dead, three-quarters of them civilians according to the United Nations, and 67 on the Israeli side, nearly all of them soldiers.

Two men aged 22 and 24 were killed in a strike on Nusseirat refugee camp early on Friday, emergency services said.

Two more were killed in an air raid near neighbouring Deir al-Balah.

The Israeli military said it struck around 20 targets overnight but did not give details.

Israeli media said the government was seeking US diplomatic help to head off the European bid at the UN to end the violence, the deadliest since the 2005 end of the second Palestinian intifada or uprising.

Washington has wielded its veto powers at the UN Security Council repeatedly in the past on behalf of its Israeli ally.

But relations have been strained over the breakdown of US-brokered peace efforts and concerns over the scale of the civilian death toll in Gaza.
The draft presented by Britain, France and Germany came after one submitted by Jordan on behalf of the Arab League had run into US opposition.

The European text urged an immediate and sustainable ceasefire, and a lifting of the Israeli blockade.

It proposed a mechanism to monitor the ceasefire and supervise the movement of goods into Gaza to allay Israeli security concerns.

It also called for Gaza's return to the control of the Western-backed Palestinian Authority led by President Mahmoud Abbas, seven years after his loyalists were driven out of the territory by the Islamists of Hams.

The text provides for the lifting of economic and humanitarian restrictions on Gaza to allow for a massive reconstruction effort.

UN chief Ban Ki-moon has pledged help to rebuild Gaza, but warned that it would be "for the last time" after three wars in six years.

But as the diplomatic pressure for a ceasefire intensified, Israel showed no sign of ending its deadly campaign to halt rocket fire by Gaza militants.

The security cabinet authorised the call-up of up to 10,000 army reservists in a new troop rotation, Israeli media reported.

Finance Minister Yair Lapid, regarded as one of the less hawkish members of the security cabinet, threatened further deadly attacks on Hamas commanders after three leading militants were killed in a pre-dawn strike on Thursday.