Donald Trump would play into the hands of the Islamic State militant group, Barack Obama claimed, in one of his strongest attacks on Donald Trump’s preparedness to be president.

Following an hours-long meeting of his National Security Council at the Pentagon on Thursday, the US president said Trump’s broad-brush rhetorical attacks on Muslims worldwide, his enthusiasm for “bomb[ing] the shit” out of Isis and his proposed ban on Muslim immigration would “backfire”.

Without using Trump’s name, Obama said that only “bad decisions” made by the US would prevent a defeat of Isis that he portrayed as inevitable.

“If we start making bad decisions, indiscriminately killing civilians, instituting offensive religious tests – those kinds of strategies end up backfiring,” Obama said.

“The reason it’s called terrorism as opposed to a standard war is these are weak enemies that can’t match us in conventional power, but what they can do is make us scared. When societies get scared they can react in ways that can undermine the fabric of our societies. They make us weaker, they make us more vulnerable.”

Yet Obama made only oblique referenceto what human-rights monitors have warned is shaping up as an over-broad US bombing campaign: aerial support for the battle to oust Isis from the northern Syrian city of Manbij, now in its third month.

The US military has two formal investigations open into two different airstrikes which monitors have said killed dozens and perhaps hundreds of civilians last month, including women and children.

Obama did not discuss the Manbij airstrikes explicitly, but said he takes “very seriously” allegations of civilian casualties.

Despite his ongoing criticism of Trump, whose judgment has come under increasing question within the Republican party after a sustained attack on the parents of a slain US Army captain, Obama said Trump would receive the intelligence briefings traditionally given to presidential nominees.

“We’re going to follow the law,” Obama said.

Obama said the anti-IS campaign was making progress in Iraq and Syria, because the group had not regained the territory it had lost in recent months.

But echoing months of assessments from senior terrorism officials, as well as the cascading Isis-inspired terrorist attacks worldwide, Obama warned that Isis would answer the loss of its Iraq and Syria caliphate with escalating, low-level terrorism, particularly through social-media borne motivation.

“What [IS] has figured out is that if they can convince a handful of people or even one person to carry out an attack on a subway, or at a parade or some other public venue, and kill scores of people as opposed to thousands of people, it still creates the kinds of fear and concern that elevates their profile,” Obama said.

He said the threat to the US mainland was “serious”, due to the difficulties of detecting attacks which involve minimal communications to a broader network, and warned that America’s lax gun restrictions enabled Isis to carry out small-scale assaults.

The dangers “of a lone actor, or a small cell, carrying out an attack that kills people is real”, Obama said, declaring himself “disappointed” when attack succeeds, “because I’d like to prevent all of them”.

While Obama looked beyond the downfall of Isis’ self-declared caliphate on Thursday, predictions of its collapse remain premature. The US military command in Baghdad conceded that an already-announced additional 560 US troops have yet to arrive at a recaptured airbase 40 miles southeast of Mosul, intended as a staging ground for retaking Iraq’s second largest city. And with Manbij yet to fall to the Syrian armed groups backed by the US, an advance on the Isis capitol of Raqqa cannot unfold.

US defense secretary Ashton Carter has set the capture of both cities, the principal redoubts of Isis in Iraq and Syria, as a priority for the US in 2016.

Obama announced no new initiatives or accelerants against Isis after his meeting at the Pentagon, but defended two security policies that have come under recent criticism: diplomacy with both Iran and Russia.

Obama dismissed a widely circulated story from the Wall Street Journal claiming that a $400m cash payment to Iran, announced in January, was a payment to secure the release of US citizens held captive by the Iranian government.

“We do not pay ransom for hostages,” Obama said, treating the account as a proxy for criticism of his 2015 accord to end Iran’s nuclear weapons program, which he challenged his critics to admit had worked.

On Russia, a nascent US proposal to expand military cooperation with the Russians in exchange for Moscow reining in support for Syrian dictator Bashar Assad has aroused deep resistance at the Pentagon. Yet Obama said he needed to pursue the plan in order to exhaust every possibility of ending one of the world’s bloodiest and most protracted conflicts, which he also argues is necessary to ensure a lasting defeat for Isis in Syria.

“If we are able to get a genuine cessation of hostilities that prevents indiscriminate bombing, protects civilians, allows humanitarian access and creates some sort of pathway to begin the hard work of political negotiation inside of Syria, then we have to try, because the alternative is the perpetuation of civil war,” Obama contended. Should the plan fail, he said: “Russia will have shown itself very clearly to be an irresponsible actor.”

While Obama has recently shown great relish for attacking Trump, at last week’s Democratic convention and elsewhere, as a demagogue, he told reporters at the Pentagon that continuing the criticism would belabor the point.

Instead, Obama urged journalists and voters to evaluate the next president on the basis of “someone with the temperament and good judgment to keep America safe”.