Spain began implementing border checks on travellers arriving from Italy on Saturday in response to border controls introduced by Rome on Spanish travellers over the migrant crisis in Ceuta.
Under the new rules, arrivals from Italy at ports and airports are subject to random checks until September 7.
It comes after the right-wing government of Italian Prime Minister Giorgia Meloni introduced checks on travellers from Spain earlier this month, following the influx of tens of thousands of migrants into the Spanish exclave of Ceuta from Morocco.
However, according to initial reports, the new Spanish checks appear to be rather superficial and symbolic, at least for EU citizens.
A traveller from Italy told Spanish state broadcaster RTVE at Madrid airport that an announcement was made on the plane stating that passengers were only allowed to disembark with an ID at the ready.
Spanish officials awaited passengers directly in front of the aircraft, she said.
The traveller said she had held her Italian passport visibly in her hand and was allowed through without any further scrutiny.
"The police officer saw that we had a passport. He didn't check it, he just saw that we had one, and that's why we were allowed through," said the woman.
The left-wing Spanish government announced the checks on Friday evening, after Italy's right-wing Prime Minister Giorgia Meloni refused to lift the border controls imposed on travellers from Spain since August 1.
Italy and Spain share no land border, so the checks introduced by both sides apply to maritime and air travel.
Migrants who have entered Ceuta have no way of flying or taking a ferry to the Spanish mainland and travelling on to Italy from there.
Meloni was elected to office in 2022 on a promise to take a hard line against unwanted migration. Elections are due to be held again in Italy next year.
According to Italian media reports, Italy's Foreign Minister Antonio Tajani, of the centre-right Forza Italia party, had been looking to defuse the dispute with Madrid.
However, the two other coalition partners in Rome - Meloni's Brothers of Italy party and the right-wing Lega party - are said to have advocated taking a hard line against Madrid.
From Spain's perspective, the Italian border controls are unfair, run counter to the interests of the EU and discriminate against Spanish residents.
Foreign Minister José Manuel Albares has described Italy's actions as "a torpedo below the waterline of European unity."
Meanwhile, critics of the government in Rome have characterized the Italian border controls as symbolic policies driven primarily by domestic politics.
Meloni's government is currently under considerable pressure from the newly founded right-wing party Futuro Nazionale, led by former army general Roberto Vannacci. His extremely anti-immigration rhetoric is increasingly resonating with much of the ruling parties' electorate.