Speaking soon after the US missile strike on a Syrian air base, Trump called on "all civilised nations to join us in seeking to end the slaughter and bloodshed in Syria".
English said New Zealand had not yet received any request for help from the United States.
Defence Minister Gerry Brownlee was tipped off by US officials about the attack, in which 59 cruise missiles fired by US warships struck an airbase at 12.45pm New Zealand time.
The target was reported to be the base of the Syrian aircraft which dropped chemical weapons in the northwest of the country earlier this week, killing dozens of people including children.
McCully said it was becoming clear that Syrian Government forces were responsible for "outrageous" attacks in which chemical weapons were used.
"These events are horrific," he said in a statement. "It is critical that the international community emphatically demand an end to this violence, and that the Syrian government be held to account."
"In the absence of an adequate response from the United Nations Security Council, we can understand why the United States has taken targeted unilateral action to try and prevent further such attacks by the Syrian regime."
Australian Prime Minister Malcolm Turnbull was briefed on the military strike this morning, according to Australian media.
The Australian reported that Turnbull received a call before 7am from White House officials to brief him on the impending military action.
Australian defence officials in the Australian base in UAE the Middle East were also alerted because of the RAAF's role in operations against Isis in Syria, the paper reported.
Labour leader Andrew Little said unilateral action had to comply with UN resolutions.
"We do not want a repeat of what happened in Iraq. New Zealand must continue to support international efforts to find a lasting peace. Until that happens the humanitarian crisis will only get worse."
Little said he was horrified at the chemical attack, "just the latest horror in this tragic civil war".
"The world did need to express its outrage over this appalling act by the Syrian government.
"We cannot let the use of chemical weapons in violation of international law happen without consequence.
"In the end that consequence must be the international community making genuine efforts to find a solution to end the suffering of Syrians."
Green Party co-leader James Shaw said New Zealand needed to engage with other countries about the most appropriate response and its possible role.
Shaw said his party condemned the use of chemical weapons, but "precipitous unilateral action" by the US was not the solution. The US missile strike was a "hot-headed" and "knee-jerk" reaction, he said.
He accepted there was frustration with the United Nations' inability to intervene in Syria, but said missile attacks would not "make the situation any less complex - in fact quite the opposite".
New Zealand currently has 140 military trainers based in a non-combat capacity in Iraq as part of the fight against the Islamic State. It does not have a military presence in Syria.
Syrian Solidarity New Zealand spokesman Ali Akil said the military strike was a step in the right direction, but it was "too little too late".
"Assad has been slaughtering the Syrians for six years. He has killed hundreds of thousands of people, he has displaced more than half of the population of the country. He has destructed the entire country and put it to its knees.
"I think destructing one airport is certainly not going to stop Assad from continuing on . . . there must be a follow on from there."
Waikato University international law expert Alexander Gillespie said the significance of the missile attacks could not be understated.
"It's huge. It's absolutely monstrous. To actually get directly involved, targeting [Syrian President] Assad - as opposed to targeting Isis - is a game-changer."
Gillespie said all eyes would now be on Syria and Russia's response.
"There are American troops in Raqqa right now, fighting the Islamic State. The question is whether Assad will attack the American troops
"And the second question is how will the Russians respond? Because the Russians have the capability to strike back.
"Best case scenario, everyone just accepts that Assad got a slap. Worse case scenario, there is a retaliation."
Trump had been ambivalent about military intervention in Syria when he came to power, Gillespie said.
"He has now put a flag in the ground."
