President Donald Trump's public rebuke of Saudi Arabia this month for its role in the conflict in Yemen was an impromptu move quickly set in motion after intelligence officials presented him with images of the deepening humanitarian crisis there, officials said.

In a strongly worded statement that surprised foreign diplomats and even key figures in his administration, Mr Trump called on Saudi Arabia to allow food and supplies to reach "the Yemeni people who desperately need it."

"This must be done for humanitarian reasons immediately," he said.

The statement on 6 December marked a striking departure for a president who has shown unflinching support for the kingdom's leadership. Administration officials say it was instrumental in Saudi Arabia's decision to temporarily suspend its blockade of the Red Sea port of Hodeida, which to critics had come to symbolise Saudi excesses in the nearly three-year-old war in Yemen.

The episode also highlights the powerful role that depictions of civilian suffering overseas have had in stirring a president who took office with a clear domestic focus and little foreign policy experience. Sometimes they have triggered sudden, consequential responses from the president.

In April, Mr Trump decided to launch Tomahawk missiles at a Syrian air base after televised images of victims of a chemical weapons attack, including lifeless children, had what he said was a "big impact" on him. It was the first direct US assault on the government of President Bashar Assad in that country's civil war, a seeming break with the administration's policy on the Syrian leader.

Mr Trump's public critique of Saudi Arabia capped a weeks-long behind-the-scenes effort as US officials sought to persuade Saudi leaders to fully lift the blockade, which it imposed on Yemeni ports, airports and land crossings after Houthi rebels fired a missile toward Riyadh's international airport 4 November. But the private entreaties from diplomats and White House aides lacked the impact of Mr Trump's action.

"When the criticism comes from State, they can discount it because that's just the bureaucracy talking," said Gerald Feierstein, an expert on Persian Gulf nations and a former US ambassador to Yemen. "But if they're losing Donald Mr Trump, they have to be concerned about the state of the relationship."

Intelligence officials highlighted deteriorating conditions in Yemen as part of the President's Daily Brief (PDB) the morning of 6 December as Mr Trump prepared to meet with Cabinet officials, according to officials familiar with the day's events. Hours after the briefing, which included imagery related to the crisis, the White House issued Mr Trump's terse, first-person statement.

Officials said the president previously had been briefed on the humanitarian crisis, but one official with knowledge of the 6 December briefing said it had prompted him to push the United States to take a harder stance. Since then, the president has asked periodically about Yemen, the official said.

"While it was a relatively spontaneous decision, it also brought to a climax many months of discussions and wrangling on the issue of how to ease the humanitarian plight in Yemen," a senior US official said of Mr Trump's statement that day. Like others interviewed for this report, he spoke on the condition of anonymity because he was not authorised to speak publicly about the issue.

As they have for other presidents, intelligence briefers frequently have employed imagery and graphics to illustrate complex conditions overseas for Mr Trump in the PDB and other briefings. Officials close to Mr Trump say the former reality television star has been particularly drawn to photos and images used in those presentations.

"He doesn't have the attention span or the patience to sit and comb through books or briefing materials," said Tim O'Brien, a Mr Trump biographer and long-time observer. Mr Trump also has made a habit, O'Brien said, of making instinctive decisions based on what he sees.

"He makes snap judgments because he thinks his instincts are unerring," Mr O'Brien said.

David Priess, a former CIA officer and intelligence briefer, said content presented in the PDB has the potential to shape presidential perceptions. "Something that is emotional, that is heart-breaking, has the ability to change policy," he said.

A spokesman for the National Security Council said Mr Trump's statement was "entirely consistent with and a natural progression" from earlier statements from the White House, State Department and other agencies.

"We have a very strong partnership with Saudi Arabia and engage regularly on a broad range of issues, including Yemen," the spokesman said.

The conflict in Yemen has presented a dilemma for the United States since gulf countries led by Saudi Arabia intervened in 2015 to contain a rebellion that they said was stoked by regional rival Iran.

US officials have been eager to demonstrate their support for Saudi Arabia, which has faced multiple missile attacks by the Houthi rebel movement. But they also have sought to distance themselves from the kingdom's management of the conflict, which critics say has been characterised by indiscriminate Saudi air strikes and inaction in the face of civilian suffering.

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The United States has provided military support, mostly in the form of weapons sales and aircraft refuelling assistance, to Saudi Arabia since the beginning of the conflict.

President Barack Obama's administration was sharply critical of Saudi Arabia's humanitarian stance, but there was a sense that such pressure might subside after Mr Trump took office, as the new president embraced the gulf nation's leaders and vowed to help them push back against Iran. Mr Trump has repeatedly defended the kingdom's ruling family, including after a controversial internal purge.

But US officials have grown increasingly concerned as the conflict - driven by the rebels and the Saudi-led coalition - has caused needless civilian casualties, led to as many as 1 million cases of cholera and pushed millions to the brink of famine.

Saudi Arabia, which blames the Houthis for the crisis, identified Hodeida as a priority because, it says, weaponry used to attack Saudi interests is smuggled through the port.

Initially, US officials made their appeals privately. At the State Department, Deputy Secretary John J Sullivan met with aid groups. White House aides engaged with Saudi diplomats.

But many officials say it was Mr Trump's public pressure that led the Saudi government to announce later in December that it would permit both humanitarian and commercial shipments, including desperately needed fuel, into Hodeida for 30 days.

Aid groups remain concerned that it will take longer than that to unload supplies and are sceptical that the port will be handed over to the United Nations as planned, raising the likelihood that the blockade may be reimposed in short order.

"As long as imports through Hodeida are threatened, the worst mass atrocity in decades remains scarily on the horizon," said Scott Paul, senior humanitarian policy adviser at Oxfam America.

Perry Cammack, a former Obama administration official who is now at the Carnegie Endowment for International Peace, said the Mr Trump administration must signal to Saudi Arabia that it is willing to withhold military support.

"President Mr Trump now has Saudi Arabia's attention," he said. "But to meaningfully ameliorate the catastrophic conditions in Yemen, he'll need to signal that his 6 December statement constitutes not just a rhetorical flourish but a real change in policy."