Spain announced on Friday it would introduce temporary border checks on travellers from Italy in response to Rome imposing similar measures after a mass migrant rush into Ceuta which killed over 140 people, mostly from Morocco.
The checks, which come "amid the persistent irregular migration pressure" facing Italy, will start at midnight on Saturday and last until 7 September, Spain's interior ministry said in a statement.
Hours earlier, Spain threatened Italy with countermeasures if it did not swiftly lift its own border checks, drawing a rebuff from Rome.
The rare spat between the European Union and NATO allies came after tens of thousands of people crossed from Morocco to Spain's North African territory on July 30-31 in an unprecedented, deadly rush.
The scenes infuriated EU members that back a hard line on irregular immigration, including Italy which imposed a one-month suspension of its visa free-travel Schengen deal with Spain.
The Spanish government slammed the measure Friday as "unfair, contrary to the EU's interests and discriminatory for the Spanish population".
Foreign Minister Jose Manuel Albares described it as "a torpedo fired at the hull of European unity."
"We urge the government of Italy to rectify, put an end to the checks and treat Spaniards like other European citizens," the government said.
He threatened "proportional measures" in response by Sunday if Italy failed to comply.
The government of Italy's far-right Prime Minister Giorgia Meloni said Rome "had no intention of revising the decision" before 15 August, when it said Spanish authorities "expected a new wave of migration" to hit Ceuta.
'Creating rifts'
"Italy does not accept ultimatums or impositions from abroad regarding national security and border control," Meloni's office said in a statement.
"Only when there is certainty that there are no security or terrorism risks to Italy, that no new wave is occurring, and that no irregular migrants are heading toward European territory, will the decision be reconsidered," it read.
Spanish police have been monitoring online rumours and social media messages about possible fresh migrant arrivals in Ceuta in mid-August, according to local media - but authorities have not expressly said they expect a repeat of last week's rush.
Border authorities would in the meantime "do their utmost to ensure maximum ease of movement for Spanish and European citizens" who are not affected by the border checks, the Italian statement added.
Italy, which does not share a land border with Spain, introduced border controls at maritime and air entry points for third-country nationals arriving from the Iberian country.
The move announced on 31 July was an "extraordinary measure" intended to "safeguard national security and prevent possible repercussions for our nation", Meloni said then.
She was among EU leaders who called for Spain to be suspended from the visa-free Schengen travel zone after the Ceuta incident, despite such a move being legally impossible.
Italy's coalition government has sought to stop the tens of thousands of migrants who land in small boats on the country's shores each year.
The Spanish government warned against "creating rifts and sterile conflicts driven by internal electoral motives" and said "we trust that pro-European sentiment, common sense and good faith prevail".
Of the 72,000 migrants who reached Ceuta, 70,000 have returned to Morocco, according to official Spanish figures, which have recorded 80 deaths.
Morocco's interior ministry has said 11 people died on the Moroccan side of the border, all but one by drowning.
Spain's leading migrant-focused NGO said the total number of deaths has reached 141, with most of the victims being young Moroccan men.
Those who remain in Ceuta - including hundreds of unaccompanied children - face dire living conditions with little access to food, water, and shelter.