Donald Trump is manufacturing a crisis at the US-Mexico border to justify his hardline immigration plans while failing to tackle the real crisis in Central America that is the root of the problem, the head of one of the world’s largest humanitarian aid groups has said.

David Miliband, the former British foreign secretary who now leads the International Rescue Committee (IRC), issued a scathing critique of the Trump administration’s handling of border issues. “The US government is failing in its most basic responsibilities, never mind as a global leader but as a local example of how a civilized country should behave,” he said.

In an interview with the Guardian from IRC’s headquarters in New York, Miliband said that Trump’s approach to immigration amounted to “disorder by design”. “The administration needs to create the evidence to justify its immigration policies – it is using the concept of crisis to create the justification for government by executive fiat.”

The national emergency declared by the US president in February to bolster his plans for a border wall were denounced by Miliband as “manufactured crisis”. He said: “By no standards of national or international precedent would you describe it as a crisis, even in the communities affected in the southern US.”

Meanwhile, thousands of vulnerable people are suffering because of the removal of US protections, slow processing of their asylum claims and cuts in federal aid, he said. “The people who pay the price for government policy failure are the most vulnerable and least able to cope, whether Americans who are on the edge or Central Americans who are over the edge. That is a great danger.”

Founded in 1933 at the call of Albert Einstein, IRC today operates in more than 40 countries, including many war zones. Its global emergency team is currently working to control the Ebola outbreak in the Democratic Republic of Congo.

Yet this week, a member of that same global emergency team was relocated to Arizona to help set up shelters dealing with the fallout of the Trump administration’s treatment of asylum seekers at the southern border. Since February, IRC has been assisting hundreds of migrants dropped by federal agents at Phoenix bus stations, where they are often abandoned with no information, accommodation or onward travel accommodations.

Miliband said that Americans should take that as a wake-up call. “It should be shocking that a global humanitarian charity has to deploy an emergency response worker to the US to help with a situation that is being created by the policy approaches of its own government.”

Miliband’s intervention comes at a time when Trump’s already volatile stance on immigration has taken a dive further into chaos. Last week he sowed panic on both sides of the border when he threatened to close the frontier entirely.

The US president has indicated that he wants an even more extreme anti-immigrant posture. He recently withdrew his nominee for director of Immigration and Customs Enforcement (ICE) in favor of a “tougher” leader.

Then on Sunday, he orchestrated the resignation of the homeland security secretary, Kirstjen Nielsen. A purge of the leadership of US immigration and security agencies is continuing.

Miliband said he was fearful that the turbulence in the Trump administration could create a vicious circle. “Bad policy already runs the risk of bad outcomes, and the fear then is that bad outcomes induce even worse policy.”

In its new report Disorder by Design: A Manufactured US Crisis, IRC sets out the ways in which Trump policies designed to reduce the numbers of immigrants entering the US illegally have backfired, contributing to the surge in Central American asylum seekers:

In 2017, Trump terminated the Central American Minors (CAM) program that helped parents in the US legally reunite with their children.

In 2018,he scrapped the Temporary Protected Status (TPS), in effect ordering the removal of almost 200,000 El Salvadorans who have been living legally in the US for more than a decade. The move has been blocked by a federal court.

The US government has introduced “metering” at the southern border that limits the number of asylum seekers who can be processed each day.

It has imposed a policy named “Remain in Mexico” that forces the return of asylum seekers across the border while they await immigration hearings. A federal judge in San Francisco on Monday halted the policy as a legal challenge makes its way through the courts.

All these policies, Miliband said, “have no justification by themselves and are deepening the problem that the government says it’s trying to tackle”.

Removing US protections also ran the danger of exacerbating violence and unrest back in the Northern Triangle countries of El Salvador, Honduras and Guatemala, which are the source of most immigrants currently presenting at the southern border. El Salvador, with a population of six million, is estimated to have more than half a million people either in gangs or associated with them, resulting in one of the highest homicide rates in the world.

Escalating violence is so severe in El Salvador that IRC recently resumed its aid operations in the country for the first time since the end of the country’s civil war in 1992. By contrast, the US government has pulled the plug – last month Trump ordered the US state department to stop all foreign assistance to the three Central American countries.

“This is a textbook example of a perverse policy response,” Miliband said. “The government has no strategy at all for dealing with the real crisis in Central America.”

He added that there were tried and tested ways of helping Salvadorans stabilize their lives and stay in their own country. “We know what works in El Salvador – there are some very good examples of how to divert people from fleeing to the US. If the administration’s aim is to have fewer people coming here, then it should be increasing aid to the region rather than decreasing it.”