A top White House official has said US President Donald Trump is “deadly serious” about slapping tariffs on imports from Mexico.

But he acknowledged there are no concrete benchmarks being set to assess whether Mexico was stemming the flow of migrants enough to satisfy the administration.

Acting White House chief of staff Mick Mulvaney told Fox News Sunday: “We intentionally left the declaration sort of ad hoc.

“So, there’s no specific target, there’s no specific percent, but things have to get better. They have to get dramatically better and they have to get better quickly.”

He said the idea is to work with the Mexican government “to make sure that things did get better”.

Mr Trump claims Mexico has taken advantage of the United States for decades but that the abuse will end when he slaps tariffs on imports next week in a dispute over illegal immigration.

Mr Trump tweeted on Sunday: “America has had enough.”

The president said last week that he will impose a 5% tariff on Mexican goods on June 10 to pressure the government of Andres Manuel Lopez Obrador to block Central American migrants from crossing the border into the US.

Mr Trump said the import tax will increase by 5% every month until October, topping out at 25%.

“He’s absolutely, deadly serious,” Mr Mulvaney said.

Mexican officials are due to meet later this week with Secretary of State Mike Pompeo in a bid to come to a resolution.

Mr Mulvaney said Mexico could take various steps to decrease the record numbers of migrants at the border.

He suggested the Mexican government could seal its southern border with Guatemala, crack down on domestic terrorist organisations and make Mexico a safe place for migrants seeking to apply for asylum.

“There are specific things that the Mexicans can do,” he said.

Economists and business groups are sounding alarms over the tariffs, warning they will hike the costs of many Mexican goods Americans rely on.

But Mr Mulvaney downplayed those fears, saying he doubts business will pass on the costs to shoppers. “American consumers will not pay the burden of these tariffs,” he said.

He also suggested the tariffs were an immigration issue, separate from the trade deal the US is trying to negotiate with Mexico and Canada.